How to enhance grout?

[Luchano]

One plumber told me that I need to enhance grouting between my tiles in my shower-room to stop leakages inside the tiles.

I have done it in my bath-room using silicone - is it a good idea? It does the job, but silicone is prone to moulds and it does not look neat.

If I should use grout - what would be the best one? How should I regrout - simply putting a new layer over the old one or by removing the old one?

 

[Luchano]

This is how I put silicone over old grout in the bathroom to protect from leaks.

 

[Luchano]

I was not sure what has caused that leak. The plumber told me that this is because of that small crack in the grout. I cannot believe it, but I see his logic since the leaks do not occur when the shower-room is not in use. Another plumber told me that it is possible that when pipes/joints are in use - they leak.