You can subscribe for any type of signals for both Blockchain protocols and Crypto Exchanges. For example, if any funds are moved from a specific wallet you will be notified, if someone places an order with specific parameters you will be notified, and so on.
Signals are very important for crypto traders. In the beginning of April 2019 an increase in the BTC price happened because of 3 main reasons, 2 of which could have been known in advance using Crypto APIs. Check the 2nd and 3rd point in this article.
Another example of how Signals can be used, is to write a program that automatically sends out an email or SMS,or sends a Tweet whenever there’s a certain pattern of incoming Crypto or Blockchain event.