What is the purpose of the straps?
Why straps? Very briefly: they hold the pad in place and prevent it from slipping, thus reducing the risk of friction and seat discomfort.
A little longer: The pants are basically a construction to keep the pad (also called chamois) in place. To do this, the shorts have to fit tightly. In cycling, it has become established to use shorts with straps. These are called bib shorts, or bib tights.