I’m not from the US but I think it’s to enable compromise.
Some lawmakers (group A) don’t want to support either a TikTok ban or aid for Ukraine. Other lawmakers (group B) want a TikTok ban and are willing to support aid for Ukraine, even if they don’t like it that much. A third group of lawmakers (group C) wants aid for Ukraine and is willing to support a TikTok ban, even if they don’t like it that much.
No group is big enough to pass anything on its own. Therefore, group B and C agree to work together to pass both a TikTok ban and aid for Ukraine.
They could pass two laws, one banning TikTok and one providing aid to Ukraine. But they don’t trust each other. If the TikTok ban was passed first with the votes of both groups, group B could betray group C afterwards and not support aid for Ukraine. If aid for Ukraine was passed first, it would be the other way round.
Therefore, group B and C bundle TikTok ban and aid for Ukraine together into a single law. Now, neither group can betray the other.
In reality, the situation is obviously more complicated. There are more than just three groups and there are additional things included in the deal like military aid for Israel and Taiwan and humanitarian aid for Palestine.