In Brazil there's what we call "preventive custody". If you're caught committing a crime, and if there is a risk that you could jeopardize the investigations (by eliminating evidence, threatening or influencing witnesses, etc.), then you are held in custody until the investigation is concluded.
I don't believe you would find something very different going on in any other democratic country.
In this scenario, are you actually charged with a crime? If not, that’s the literal definition of being jailed without trial.
Many (most?) democratic countries impose strict limits on how long you can be held without being charged. In the US, for example, you can only be held for 72 hours — at which point the police must either charge you or release you.
Sure, you're actually charged with a crime. And the kind of limitations you talk about do apply.
Even in situations where you could be held in jail, there's a tendency to let you go unless it is impossible to prevent you from jeopardizing the investigations by any other means. For instance, if the only real worry is that you flee to another country, you might have your passport confiscated rather than being held in jail. Likewise, if the worry is that you can use your influence to make others do stuff for you (stuff that jeopardizes the ongoing investigations, I mean), then you might remain at home, under surveillance, and so on.
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.
> where it's possible to hold someone in jail before trial
Honestly, what did you expect? "Pretty please turn up for the trial and not kill anyone else while we're waiting?" Every country allows this if the crime was severe enough or the person is likely to be dangerous to others in the meantime. Usually there's a threshold to do that, but it's going to happen.
The threshold, at least in the countries whose laws I'm familiar with, isn't just "will they turn up for trial", but also "are they a risk to other people or are they likely to continue breaking the law". And, to be honest, that makes sense.
