Question

What purpose is served by the pits and pores in xylem and phloem cells?

Answer 1

Those are on the side walls of xylem! They allow side wise flow of water. In cases a bubble blocks a xylem, through side pits the water bypass.

In the case of phloem, if the sieve tubes (which are metabolic active and full of cytosol) did not contained pores in their tops and bottoms it would have problem in transport.