PCR is a process which requires the mixture of targeted genes, free nucleotides, taq polymerase, and DNA primers, to be heated up to 95°C in the process of denaturation (the splitting of double-stranded DNA into single-stranded DNA). The mixture will then have to be cooled to 54°C for annealing to occur (where DNA primer attaches to DNA by complimentary base pairing at the 3' end flanking the target sequence of DNA/gene). Then, the mixture will have to be heated up again to 72°C for extension of new DNA which is synthesized based on complimentary base pairing (A-T, C-G) by taq polymerase.
This process must be repeated many times. Thus, it is good that taq polymerase can withstand such temperature changes, especially to 95°C and above, so that it is not necessary for scientists to continually replace the denatured polymerase after every round of PCR if the polymerase is not heat-stable.
Hope this helps! :)
