GreatArtist wrote:
Why do you have to lay down tiles with mortar and then put grout
between the tiles?
Why can't you just put mortar between the tiles?
Is grout more waterproof than mortar?
Can you use mortar as a substitute for grout and then put a penetrating
sealant on top of the mortar?

I might be wrong, but I don't think you can put sealant on mortar. The
obvious answers are that

1. Mortar is messy
2. Grout can be colored to people's wants and needs
3. Grout is much more forgiving
4. Grout does not stick to mortar

Mortar is formulated to bond masonry units together. The main
ingredient of mortar is Portland cement. Grout is not mortar and not
concrete either. It is in between like a soft concrete. Grout has a
certain fluidity because it must fill in narrow spaces well. Mortar
does not have the same consistency or fluidity as grout. Plus, grout is
made of finer materials that are similar to mortar, but the makeup of
grout is different. Mortar just doesn't flow the same.