You indictor is an indication of your intensions so that the other drivers are not surprised when they see you car move from the expected path. If you are only a road with light traffic then yes, first adjust your speed so cars are not in your blind spot and then indicate.

But, if you are in tight traffic then a gap will unlikely open up. In this case you use your indicator to signal your intention, and wait until somebody chooses to let you in - making sure they have noticed you before moving in.

A car that beeps at you just because there is a car in your blind spot when you indicate is not suited to Australian road rules.