Multiple concentration checks at once.

[Krikit]

Hypothetical situation:
Casting defensively (DC: 15+spell level) while Entangled (DC: 15+spell level)?

Do the DC's stack? Or are they rolled separately? Or do you take the worst of the two?

[blahpers]

Gotta pass 'em both. If someone readys a magic missile in response to your casting, that'd make three checks.

[Perfect Tommy]

Not to mention if its raining and you're on a boat...

[John Murdock]

at least against raining and being on a boat the feat uncanny concentration helps greatly since it negate the need to make the check against those and it give a +2 to concentration check