Fees are important because they reduce the performance of your pension, and over the long term even small differences in fees can have a big impact on money you end up with. The funds used for auto-enrolment can charge at most 0.75%, though other funds that you can choose from in your pension may charge more.
You should carefully compare the fees charged on any old pensions with other pensions available – where cheaper options are available, you may want to switch.