A recommendation letter where professor mentions he doesn't know student very well

I had a colleague who asked a professor to write a letter of recommendation on his behalf. The professor agreed and wrote 3 letters for this colleague. The colleague had an interview with one of the colleges he applied to and he came to know that the professor wrote that he doesn't know him very well.

My questions are: why would a professor agree to write a recommendation in the first place?! And what is the effect of this letter on one's application?

I asked him to write one on my behalf, and he might have written similar statement.