Today I approached a give-way at a more major (but not very wide) road, and noticed a bus approaching. Because it was indicating to turn my way, I realised that I should stop short of the give-way line to allow it room to turn across the front of me. If I had stopped right at the line, we would have needed to reverse the priorities, which can be confusing for other road users.
Similarly you never know who might benefit from your turn signal (a pedestrian, a cyclist) which can advise them what is happening. The pedestrian might intend to cross the road where I am turning, etc. So the drill is to observe, signal your intentions, and make the manoeuvre. There may always be another road user that you didn't notice.
You also mention side indicator lights: you might be almost at the junction when someone looks at you, and can't see the front ones. Those lamps are also further out than the rear indicator lamps, so they are more visible in traffic from behind, and from the front when intending to pull out from a parking spot.