I followed people who made things that I liked to get the latest news about those things. I participated a lot in group watching of trash TV, that's what I miss most. Mastodon doesn't have the numbers yet to do that regularly. Also, customer support for almost any company, where else can you reach someone as easily as on Twitter?

Oh and very local stuff happening right this second. A few years ago, the sirens in my area randomly went off and nobody knew wtf was going on for at least an hour. Turns out they were misfiring and nothing was going on, as we eventually learnt from an official announcement. Twitter was immensely helpful for communication in that situation, not least to see that everybody else was just as confused as I was.