The gemara enlightens us by telling us that מחשבה טובה מצרפה למעשה, a good thought (if one entertains the thought of fulfilling a mitzva) is regarded as a good deed. The word מצרפה doesn’t seem to be the right fit in this context as it literally means to combine. Why doesn’t the gemara instead say מחשבה טובה נחשב כמעשה, a good thought is considered like a good deed?
R' Zusha of Anipoli explains that many times a Jew performs a mitzva out of rote. Consequently, in such a situation, the מחשבה, the proper thought and intent, is lacking. However we do have the מעשה, action. On the other hand, at times, a person has the מחשבה, thought, to do a good deed but for some reason he is unable to execute it. What comes out is that one Jew has the מחשבה while the other has the מעשה. Each one is like a half a mitzva. What does Hashem do? He takes these two and matches them with each other. As a result, together it is a complete mitzva because when the two are combined we have a מחשבה and a מעשה. This is why the term מצרפה is employed here.
