Generally after a root canal treatment dentists recommend a cap . You have undergone this treatment in order to save your tooth , now after a rct the tooth is generally brittle (for lack of a better word) & has a tendency to break easily when u bite something hard .If your tooth breaks while eating something & you have to lose it , it beats the purpose of doing this treatment, doesn't it ? So, by putting a cap you r actually prolonging the life of your tooth by a few years. Hence for good reason your doctor is suggesting it - however there is no great emergency to do it , but no real reason to postpone it either . Good luck :)