Because there's an ongoing investigation/legal process and you need to prevent something bad from happening. Typical examples are: the risk of you running away to another country, the risk of you jeopardizing the investigation in any way (say, by threatening or harming key witnesses), the risk of you getting killed by accomplices who are on the run, and so on.

Because you haven’t been convicted? The way it should work is that authorities are only allowed to jail you before conviction under an extremely narrow set of circumstances, such as posing a credible risk to others.