I am not quite sure that I fully understand your question, but I'll attempt a response. You can post follow-up questions if you have them.
1. This pen is used for signing important contracts.
2. This pen is for signing important contracts.
Questions that could be asked about the pen if we did not know the information above:
1. For what is this pen used? - very formal, and rather old fashioned.
aaWhat is this pen used for? - normal, natural.
2. For what is this pen? - Unnatural. Not acceptable English.
aaWhat is this pen for? - normal, natural.