(The OP is a high school junior interested in Marine Biology, from her profile. And OP, I only mention your age as rebreathers are rather advanced diving with increased risk.)
I can likely answer the Marine Bio part, at least until you're a graduate student. You don't need a rebreather. You do need to be an AAUS Scientific Diver. Beyond that, it depends on the needs of your research. I'm not a Marine biologist, but I'm a non diving related Ph.D. candidate who helps with our scientific diving program.
As was said, rebreathers are great for deep or long dives. They simplify logistics for deep or long, at a cost of being more complex than something that gives gas when you inhale, if some is still left in that tank.
Anyone working while breathing compressed gas underwater as a scientist, even at university as graduate or undergraduate students, must be certified as Scientific Divers. Most universities, with a Marine Bio program, have a program to train them, under the umbrella of AAUS for those in the US,
American Academy of Underwater Sciences. It is a OSHA regulation thing,
History, other countries have similar requirements.
If you want to dive on a research project at school, you need to be a 30' Scientific Diver. Later, you would gain experience and training for 60', 100', ... and whatever mixed gas or rebreather you need for your research. In terms of Marine Bio, do not worry yet about rebreathers. Get solid open circuit dive skills, Buoyancy/Trim/Propulsion(frog, reverse, and helicopter kicks), First aid, rescue, dive master level knowledge of diving physics and physiology. These are all parts of the basic scientific diver program. (And all just tools for your cool work and interest on X.)
Search for 'Scientific Diver' and the web sites of universities near you. If your university does marine bio, they almost certainly have a scientific diver training program. Check how often they give the class! Set up your diving so you are ready for it. The programs are all similar in terms of coverage as they all must meet the same standards.
There may be 'junior' programs near by. I know there is a group in Atlanta that teaches younger students in a 'let's do science' frame work associated with people from Georgia Tech's scientific diving staff.
Few, if any, would expect you to be a scientific diver before university. I would be shocked if any prioritized it in an application over what science you have done, not-yet-observed-on-scuba. It is a tool you will need, but one rare to get before university. Past that, rebreathers are something likely only a few graduate students use, if any. It really just depends on their research, its funding, the trade off of the time to get them trained vs getting interesting results some other way, and the university Diving Officer approving the dive plan using it as a safe procedure for those divers on that research project.
There are others here,
@tbone1004, with more experience in scientific diving. I have all the core recreational certifications, but I list 30' scientific diver as my highest.
As a minor secondary recommendation, most of the scientific divers I've seen dive with BP/W, as it is adaptable with lots of places to hook equipment etc to. And scientific diving starts looking a lot like technical diving. You at least need to hold position and orientation over what ever you’re trying to look at, while taking notes and not destroying it or other stuff near by.
Welcome to science, and more diving. There is lots of cool stuff to see and study out there.
Edit: I did not quite answer your question. If your research, post grad school, takes you to study below say 200', a rebreather will be very handy, or essential. If it involves spending 10 hours a day at 30', a rebreather may be handy as well, and quieter thus less disruptive of the fish behavior, which might affect your research results. But there is a lot of research outside those two. I'd say if you are very interested in rebreathers, pending parental advice, picking them up in the late stages of undergraduate or in grad school would give you a *few* more options for research. But keep in mind their increased complexity. If you want to study X in remote part of the world Y at 60' as a grad student, the consumables for open circuit, air or Nitrox in a tank, is all over the place for the recreational dive industry. The consumables for rebreathers, O2 gas, dilute gas, CO2 scrubbers, are available but less so.
And the rebreather, or open circuit rig, is really just the way you get to work and part of your field kit while there. It's what you need to get the job done most efficiently. Your skill at being at a position and orientation in the water column needs, ideally..., to be unconscious so you can focus on the science. The diving-as-background-task is a reason why they do this in buddy teams, possibly with extra safety divers.
I disagree that a university student would never use a rebreather, based on conversations with our staff about snorkels for observational studies v.s. a hard 5-15' bottom with pure O2 rebreathers. An experienced careful rebreather young grad student seems safe on an existing 30' project that used rebreather to not disturb behavior, and certainly at 20'.
