In favour of watching in theatres:
-Experiencing the film at the time of release
-Seeing cool trailers for upcoming releases
-Watching it in a room that was built for the sole purpose of watching films, with better sound and picture quality than you'd get at home
-Having no distractions - just be in the moment of the movie
-Being in a room full of people who are excited to watch the movie, and willing to spend money/travel in order to do so
-Possibility of special events, midnight screenings, 3D/IMAX, costumes, director/actor appearances

In favour of watching at home:
-Free, or much cheaper than theatres
-Can watch at your leisure, with pauses for bathroom breaks if necessary
-Can eat or drink whatever you want
-Trousers are optional
-Very low risk of being interrupted or shot at