As with any operation, you need to have the appropriate facilities and infrastructure to cope with the maximum demand such as:
- commuter trains that are used for only a few hours in the morning and the evening rush hours.
- peak daily/seasonal electrical/gas demands
- peak road usage demands that fall off substantially during school holidays
- peak visitor demands at outdoor attractions such as zoos, recreational fishing sites, national parks and gardens
All peak requirements involve the provision of extra facilities/staff.
By charging/charging extra at such times it can help spread the peak load and/or compensate/offset some of the incremental cost, "wear and tear" and environmental damage caused.
I see no real downside and potential significant upsides for charging extra at peak times, provided it doesn't just deter people from using the facilities.