The Government depends on a majority to get leglisation through Parliament.
The more MPs it has, the easier it is to get its stuff through.
Theresa May has had a small majority this Parliament. That meant backbenchers (her MPs who were not in Government) could tell her that certain things they want to do are not good and she should back down or change them.
The school academies policy and Grammar schools was one. As was the National Insurance issue. She had to listen to her backbenchers.
If she gets a large majority, she can get controversial policies through.
We had a coalition - and they held the Tories in check on some things . But not everything.
Basically, look at the past results for your area. If you want the Conservatives to win, then vote for them. Chances are they are going to win anyway overall.
If you DO NOT want the Conservatives to have landslide, then look at your constituency.
Some people say you should vote for the person you want to win.
Some say you should vote to stop someone who you don't want to win.