I'm going to write a short answer and skip the part "what is it?" (what does it form?), get to the "Why (he used verb+ing form)" part, and put it succinctly.
It's because in English...
you can't have two main verbs in the same clause.
(I wish I could make the font bigger.)
(NOTE: That's technically not completely correct. A more correct version would be "you can't have two main verb groups" in the same clause, but that wouldn't be as succinct as I want it to be.)
In other words: it's always one clause, one main verb (group)!
There are many ways to work around the problem, when you want to put more verbs into the same clause. One common way is to join them with a conjunction (as in G. Ann - SonarSource Team's answer), though this would change the meaning. Another common way is to turn other verbs into something else, and the easiest way is to turn them into participles (verb+ing), just like in your example. (See more details in Araucaria's answer.)