The allure of a highly used social product seems to always tempt FAANG companies. The same reason Google, Apple, Amazon and Facebook would want this is why Microsoft wants it too.

There’s probably something to owning regular attention & data on a majority share of US smartphones, regardless of the specific platform or app.

TikTok presumably hits a broad audience that Office / Outlook / Teams do not.

Perhaps you could evolve another mobile OS out of TikTok that is not Android or iOS. MicroSoft failed to get traction with its two previous mobile OSes (perhaps because they were basically offering reworked Windows).

Companies can sometimes act as VC firms and make acquisitions purely as an investment, a growth engine they can use for their core businesses.

The same reason why they bought Minecraft and tried Mixer: to acquire the next generation of technology consumers.