I actually thought that Zimmerman might have been arrested

1) to assure he wouldn't flee

and
2) to protect him better than he would be outside jail.

As i said previously, as described to me by a Broward Country sheriff friend of mine, he said that often you delay arrest until evidence can be gathered. Because once you arrst them, there's basically a clock ticking, and if you can't present your case in time, you're forced to let them go and lose the ability to prosecute.

But since they waited 2 months to make the arrest, one assumes they've gathered the evidence they feel is needed to make --or at least attempt-- a prosecution.