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Because I'm part Luddite and believe in redundancy, even though MacDive is backed up to iCloud.
Written log has all my notes on conditions, weighting, any issues, etc. I like MacDive to see the actual dive profile and stuff like that. So it's not really a duplication of effort when I'm using them for two different purposes.
I'm also very glad I've continued to log dives over the past 24 years, because it has been really fun to look back at them.
What do you want it for? I.e. if your goal is to save you copying stuff from comuter screen, then you want one that can download data from your computer. Etc.