`git add -i` is linear, you have to address files and chunks in order (there are workarounds for this, but they're clumsy). `git add -i` is a bit like using a typewriter, whereas `tig`, which lets you address files and chunks non-linearly, is like using a word processor by comparison. Edit to your hearts content.

Tig is just faster, staging and unstaging take a fraction of a second.

I've used Tig daily for years (about ten) and basically use it for for quickly browsing the log and changing what's staged - probably less than 5% of its features.

Everything else I do via the command line with ZSH's excellent git completion.