A house church is basically a meeting at someones home. Its not a public meeting. Its just like a group of friends meeting. You have to be invited to join.
Why a house church
Because of that the great majority of christians are broadroaders according to Jesus Christ Mt7:13 and we are advised to depart from them. It makes it virtually impossible to have any larger churches than house churches.
Because in churches generally you are not allowed to exclude people. They are tax exempt buildings so they have to abide by government regulations. They are basically public buildings and anyone can join.
House churches on the other hand are private meetings so you are allowed to exclude people.
So a house church is the only way to realize fellowship between christians.