Then why the constant banging on about the towel? What is it about the towel that means DP HAD to have noticed it?
It means she was in the shower (or bath... she's not sure!!). This means there's a time when the kids are unsupervised. I'm not saying that's not normal. Single parents, for example, take showers. But it does mean there is a potential for an accident to happen.
So this should be a time period of potential importance. The Police need to dig a little deeper which is what the LP do. Though they miss the opportunity to ask DP why he originally said KM and GM were both there and that it was "17:00" (this is thought to be a UK Police transcription error - they mean "7pm"). Either way this period of time is of vital importance. What time to and from was GM playing tennis? Why did KM originally say she had a bath when GM was there - but later it's a shower when he's not there? Did DP see just KM or both parents? If KM was alone with the kids - what were they doing just before she got in the shower? etc etc
The episode unintentionally, IMO, also blows a hole in Mitchell's attempt to give KM an alibi for the "missing six hours":
The friend insisted: "Kate was never alone with Madeleine that afternoon. There were always other witnesses present. These details were all given to detectives during police interviews, so they know what Kate and Gerry said happened that day."