We give envelopes to our members for a couple of reasons:

First, it lets members (or guests if they wish to use the blank envelopes located in the pews) the ability to specify where they would like their contributions allocated, such as the general fund, denominational ministry shares, the faith promise fund, the benevolence fund and other funds.  

Second, using an envelope with a number assigned to you provides a record for tax purposes when claiming a charitable deduction on your income taxes.  Members receive quarterly and annual statements reflecting their total contributions.