Why should we search for life in the universe? Such an amazing complex question.
There are many reasons. For me the most compelling two are 1) to find out how our own planet works by watching other worlds, both older and younger than ours, to piece together the life story of an Earth and get a first glimpse in our potential future, 2) to find out if we are alone in the universe or not.
Listening for civilizations that build technologies like ours is one very specific search approach. A planet needs to be in the radio age - or if we switch to laser light signals - in the laser light signal stage. Even with billions of other rocky worlds out there, it is a fascinating question how far away the next radio-age world would be? A find would of course be amazing. And start a whole new age of trying to understand such a message. Listening for such signals just got some private funding from Yuri Milner.
How many other forms of life could be out there? It is nearly as complex a question as to why we are searching.