ok, it shows creatures with evil intent but how can i benefit from this spell?

It is in fact the only game of the Infinity Engine series where Detect Evil is of any practical use (as they came out of the box).

Anytime when you are talking to someone that might or not have an ulterior motive to send you in a quest or give you some doubtful advice, you can use the spell on them.

Icewind Dale and Baldur's Gate had very few places to use the spell on, while BG2 had none. BG2 even had wrong alignments set up all around the game, which could make someone which relied on Detect Evil commit quite a few mistakes (this was fixed on mods and user fixes).

It has some uses, as Links mentioned: if you're ever dubious of an NPCs intentions, Detect Evil. As GI Joe said, knowing is half the battle. Is that damsel in distress really in distress, or is she just posing as bait for her friends who want to rob you? Planescape: Torment has more of those opportunities than many other games. That said, a high wisdom score (which any TNO should have) will let you notice most of those in advance, and worst case scenario, you can probably just pummel the ne'er-do-wells anyway. It's not useless but it's FAR from essential.

thanks guys.
too bad that the only priest (fall from grace) is acquired quite late-ish in the game in order to use this spell. hmmm, i would have liked to use detect evil on mar or the bosses in the alley of the dangerous angles—for instance.



on a related note:
is Protection From Evil any useful and how does it actually work?

An admitted necro, and it's actually to do with whether Detect Evil in BG2 is ever useful: according to http://www.gamebanshee.com/baldursgateii/quests/paladin.php (spoilers for the Paaldin quests) there is in fact a use for it.