Basically the 10% tithe is no longer a requirement. That was more OT law. Now we are free to give as we feel led. The 10% concept is still taught by some, and some teach it as a good guideline. Such as if you're not sure how much to give, then 10% is a good start. The key is, giving with a cheerful heart. Whether all you can afford is $5 or if you can give $1000. If you aren't happy to be helping, it doesn't matter how much you give, it won't honor God.
The reason for tithing is just like any other organization. It needs money to run. Paying the pastor, building maintenance, staff, improvements... not to mention hopefully your church is active in other ways such as community programs, or missionary work. All of these things require money, so it makes sense the congregation help fund these things. As well as serving bodily in them as well.
