Launch Of RNLI Bridlington's Shannon Lifeboat 13-22 RNLB Anthony Patrick Jones 

Because of tides.

Because the tide goes out a long way meaning the lifeboat would be stranded on the sand for half the time.

If I remember correctly the harbour would not float the boat at low tide also its not the best entrance in bad weather

if it's an immediate launch, e.g. a person in the water they gonna gun it and get to them asap, this they are extra safe so they don't miss anything

And they have a smaller quicker boat to launch if someones close and needs IMMEDIATE assistance

Because of local conditions you cannot keep a boat afloat, it may not be the quickest way to launch (it goes alot faster in an emergency) but its alot better than not having one, generally this method is used if there is no harbour or the harbour drys out at low tide or in bad weather the harbour entrance isn't safe

How long do you imagine it takes to get a rescue helicopter airborne, or a boat to travel 20 miles from the nearest deep water harbour?

Battery has gone

Not deep enough to float the boat at all states of the tide and the entrance can be dangerous in gales

No rush when on a training exercise.