A titular see is a flock-less see employed by popes to provide a canonical basis for episcopal ordinations for Vatican curia.
Also it's done for Auxiliary Bishops as well. For example in my diocese, Bishop Gonzales is the Titular Bishop of Lamphua, Bishop Holley is the Titular Bishop of Rusibisir, and Bishop Knestout is the Titular Bishop of Leavenworth (suppressed as a geographic See in 1947 and restored as a titular see in 1995).