Steven P. McNicoll writes:

You can do that under VFR with even broader limits on altitude and the
additional advantage of altering your route at your discretion.

But without the guaranteed services of a preexisting IFR clearance. You could
ask for a pop-up IFR clearance if you have to reenter IMC, but there's no
guarantee that you'll get it, or at least no guarantee that you'll get it
immediately. By staying IFR you don't have to worry about that. Being
VFR-on-top lets you keep the security of IFR while allowing a bit more
VFR-style flexibility.