Funds generally call capital from limited partners (LPs) in several tranches in order to optimize fundraising - it's simply easier if upon admittance to the Fund, an LP only has to wire say, 20% of its full capital commitment rather than 50%.
Sometimes, an LP may wish to send its full capital commitment ahead of time, as "pre-paid" capital, often for convenience. While this is possible, it creates more work administratively and on the banking side to track and silo the pre-paid funds separately - so it's not something to recommend or broadly offer in most cases.
IMPORTANT: Capital calls will "draw down" on pre-paid capital on the same percentage/schedule as the rest of the LPs. However, any pre-paid funds that have not been called CANNOT be used for any purposes, including deployment.