Why do we need lemmy and kbin? Why do we need various linux distros? Why do we need different office suites?

Having a choice is always a positive, more so if those alternatives are compatible like various activity pub projects.

I don't have an up to date feature comparison between Mastodon and Misskey but some of the things that differentiated the latter were:

• better threading
• improved markdown support (animated, also could render LaTeX formatting if I'm recall correctly)
• calendar
• drive
• emoji reactions (this replaces "likes" for example)
• trends
• tips for newcomers

Keep in mind, these are from few years ago so some things may have been changed or added to Mastodon (I'm not too familiar with Twitter like projects in general).

There's also already mentioned focus on Japanese audience - some of it comes down to already mentioned features (I feel like western social media aren't as... animated? as Asian or at least Japanese ones), as well as culture (I think this picture of one of the tips is a good start.

I'm sure someone more familiar with both can come up with a better comparison but I think that the main point of just having an option is the most valid one.