What is a Bank and why does the PICmicro use them?

[The PICmicro only has 7 bits which it can use to identify a register file. This means that there is only space for up to 128 file registers. The PICmicro can make available more register locations by switching banks. Each bank is an individual set of register files. The 16CF84 has two such banks which are selected using a bit in the STATUS register]