¿estos adjetivos son sinónimos?
¿Cuál es la diferencia entre ¨praktisch¨ y ¨nützlich¨?
praktisch = práctico nützlich = útil
They can very be used interchangeably and it comes down to personal preference on when to use which. The difference I can think of is that "nützlich" comes from "der Nutzen" (the benefit) and something that has a purpose (einen Nutzen haben) is useful/beneficial (nützlich) - so something that is nützlich serves a purpose. A thing that is practical may potentially lack a certain purpose.
This explanation is obviously very farfetched and for all things I can think of both words can be used interchangeably.