The goal is data collection. Even if they do absolutely nothing with TikTok, they are still getting a massive amount of data on TikTok users through the purchase.
We have seen this with Skype and LinkedIn. They are acquiring an exisitng user base and the data that goes with it. Microsoft was even on of the early investors in Facebook at a point when Facebook had no proven business plan, but had many users.
It is arguable Microsoft has been trying to transform into a data collection company. Aside from these acquisitions, we see them aggressively pushing their Windows users to accept telemetry, automatic upgrades and a view of MS software as a subscription service.
Some would argue that Skype was a controversial company when Microsoft acquired it. One could argue that MS solved a problem for those who were uncomfortable with the growth of Skype. After the acquisition, Skype was no longer controversial (except perhaps to a small number of technically savvy users who disliked the architectural changes MS made).
Perhaps the same could be said of TikTok. For some, TikTok is a controversial company. If MS acquires it, will TikTok continue to be controversial? Let's wait and see.
When Microsoft aquires a company it does not necessarily mean that whatever the company has been doing will continue. For example, the company behind the T-Mobile Sidekick, Danger, was perhaps the first cloud-syncing smartphone and its founder went on to create Android at Google. Microsoft acquired Danger. It did not then become a MS product. Most people have never even heard of Danger. The OS, which was NetBSD-based, was something MS could not work with. Did we get a next-generation NetBSD-based phone from MS? No, we got s series of successive failures to create a Windows smartphone instead.
Microsoft as a dependable steward of technology created by others is still unproven. Look at Internet Explorer, originally created using the Mosaic code from the U. of Illinois. After so many years of work developing it as their own browser, they now abandon it for Chrome.
Perhaps this is why their acquisition of Github has some people concerned.