The biotinylated selector probes bind to only the parts of the DNA library that are wanted and therefore you are left with a smaller library enriched for only the regions you want, which can then be sequenced.
The biotinylated selector probes bind selectively to the regions of the DNA library that were chosen to be where the recurrently mutations occur in the given cancer type. In this way you are left with a smaller library that is enriched for only the regions you want, which can then be sequenced.