I know there are similar questions (e.g. this), but they're written in German. I've come across this text:
Langsam biegt ein Lastwagen um die Ecke.
Why isn't it:
Langsam biegt ein Lastwagen um die Ecke ab.
And when should "biegen" be used rather than "abbiegen" and vice versa?
Consider ab- in abbiegen as away. You "turn away", so to speak. That is, you turn away from the road, or plainly said: You leave the road.
The opposite is einbiegen where ein means onto and you turn onto the road.
The verb biegen, however, doesn't indicate the direction it just states that you turn or that something (e.g. a road) bends.
As this isn't really telling a lot, you normally describe the kind of turning with a prepositional phrase. A very common one is "um die Ecke".
Er biegt um die Ecke.
In respect to this very meaning, I don't consider biegen a common word in comparison to ein- and abbiegen.
Here are a few further examples taken from the Duden. The prepositional phrase is put in italics.
Der Weg biegt um den Berg.
Der Wagen ist eben in eine Toreinfahrt gebogen.
Sie bog rasch zur Seite. (=> Sie wich zur Seite aus.)
Note, "Langsam biegt ein Lastwagen um die Ecke ab." is correct, too. But it contains redundant information and, therefore, you won't hear it too often.
"Abbiegen" does always imply a crossing or junction - You leave one street into another.
"um die Ecke biegen" doesn't necessarily - It could just mean we're on a curved road.
The 1st sentence is full of suspense, somewhat similar to what you get in music with unfinished cadences. There is more to come...or is there?
The 2nd sentence is complete and a story could finish there and then or move on to another chapter.
biegen and abbiegen are not the same. One is bend and the other is turn. It's just because the prefix has been moved to the end of the sentence that biegen (biegt) is on its own (apparently). You can separate the prefix and the word to add descriptive things, i.e. biegt "um die Ecke" ab.
