The Serrano peppers are still hot without the seeds however removing the seeds from the Serrano peppers makes them a bit less hot than they would be without the seeds.
The seeds add more heat to the Serrano Peppers so if you remove the seeds from the pepper it will be a bit less hot but still pretty hot.
The Serrano Peppers are commonly used when making salsas and sauces to add a considerable amount of heat to those salsas and sauces.
If you're using the Serrano Peppers to make salsa or sauces you should roast the Serrano Peppers before using them to make the sauce or Salsas.