Salt prevents the fermentation yeast. If grains of salt reach yeast cells, they will damage the cells. This is why you want to dissolve the salt in water prior to adding dry yeast to it.
If you add salt to flour when you are just starting to 
mix the dough, the fermentation time will increase.
Some professional bakers add more salt to their dough on warm days because fermentation process is quicker and is less predictable compared to fermentation occurring in cold weather. Obviously, you can only add a somewhat small amount of salt to influence the process of fermentation without having the salt change the flavor profile of the flour.
When added to gluten, salt makes it stronger. This leads to the dough being tighter. Salt also makes the dough more elastic, more resistant to breaking. If you need to make difficult dough, caused by soft water or aged flour, workable, you can do so by adding some salt to it.
However, tightening gluten also means that kneading will become harder. Usually, mixers add salt at the end of the mixing process. This minimizes the amount of work during the process.