This file defines which spells are given to priest-types on level-up. Priests gain spells SPPR[1-9][**] where ** is the column from this file.
This file seems to have no effect on mages.
In BG1: TotS we have:

2DA V1.0
0
            1       2       3       4       5
MAGE        19      19      17      11      8
PRIEST      10      14      12      8       6