Question

what is the use of star icon present next to the slide in slides pane​

Answer 1

Answer:

If you observe closely, you may see that these stars are shown only against some slides and not all of them.

The stars are just an indication that the particular slide has custom animation. That was you can quickly see at a glance if a slide has content that is animated.

The same star symbol is also shown when the slide itself has any slide transition set.