Why do we need to 'estimate' the PSD I've read about the PSD of one OFDM symbol for instance being an estimate of the PSD, but how can it be an 'estimate', when it literally is the PSD of that OFDM symbol.
What is actually being estimated? I get that the OFDM symbol is supposed to be a single record of a WSS random process, and therefore its an estimate of the PSD of the random process because you can't calculate the expected value. But why would you want to calculate the PSD of 'the random process' and not just a single OFDM symbol – surely that's the granularity of focus.
why would you want to calculate the PSD of 'the random process' and not just a single OFDM symbol – surely that's the granularity of focus.

Because you usually don't care about the single symbol as much as you care about making statements on e.g. the channel in general. So, you estimate the PSD.