There are lots of reasons for identifiable envelopes when giving to the church. First of all, the service the parish can provide from a tax standpoint is very important. I forget the exact figure, but I believe if the amount given to a charity is above 400, you need an acknowledgement from that charity that you gave the money and received no goods or services in return. By far, the easiest way for a parish to do this is with an envelope system.
The parish uses the envelopes for many things. For large parishes, it is a good way of “cleansing” its registered parishioiner database. People move and do not tell their parish all the time.
Also, we need to remember that supporting our parish/local diocese is not charitable giving. It is an obligation we have as catholics and as such there should be no expectation of complete anonymity with our normal sunday contributino. Not that it should be made public, but there is nothing wrong with pastors having some level of evidence that a parishioner is fulfilling his obligations.
Finally, and most importantly, using an envelopes as opposed to reaching in their wallet/purse for whatever bills are available at the time, makes us consider how we are living up to our responsibility to our community on a regular basis. As such, all tother hings being equal between two families, the family which regularly uses the envelope gives more and consistently that the family who simply digs out their loose change.
Use your envelope always, even if you are poor and can only afford a couple of bucks a week. It is a good habit. There are a myriad of good reasons to do so.