what is that little blade on the back of a pipe cutter for?

Answers:
Reaming out bur's that form from the cutter crushing in that last millimeter or two of the pipe. If you leave it "unreammed" it can affect flow, and can also result in pieces flaking off and ending up plugging your showerhead or scoring your valve seats and disk.

Other answers:

how can i get rid of mold smell in my basement?

Renovate/ Redecorate my house?

Where can I get a spirit level with an offset bubble?

Where can I volunteer against hate crimes in Omaha, NE?

How can i made soap bubbles without glycerin?

Do windows (double glazing) come in standard sizing? If so what are these standard sizes?

How do I make a quilt?

can i have a pice of candy?

Cost to alteration rough-in stopcock contained by a shower already installed?