In the beginning of the New Testament Church, we see many believers gathering together and fellowshipping with each other. We can understand the importance of interacting with other Christians by examining the following Short passage of scripture.
I long to see you so that I may impact to you some spiritual gift to make you strong - that is, that you and I may be mutually encouraged by each other's faith.
When we connect with other believers, God causes something to happen in us and through us that can only be understood by having the experience. When we gather together, one person may have the gift of healing, another the gift of discernment and yet someone else may have a prophetic word.
Fellowshipping to together causes these various spiritual gifts to work in unity together like a car engine with all of its parts properly in place.
A greater understanding of the importance of fellowshipping together can be understood by reading the entire chapter of 1 Corinthians 12. In the chapter you will see that there are many parts to the Church body being whole and functioning together.