No need for an EV or solar or batteries or whatever to get some of these rates BUT there's a risk that you may pay more than average at peak times. We have Octopus Agile where rates change every 30 minutes, can fluctuate a lot and occasionally actually go negative - today we had a few hours where we were paid a few pence per kWh for what we used!.
During the peak useage period - usually 4pm to 7pm rates can sometimes exceed the typical flat rate. In the last month, one day actually peaked at 44p / kWh briefly - the highest I've seen. Octopus Tracker doesn't fluctuate so much within the day but still varies from day to day.
Note that you need a smart meter for this stuff but that applies to the EV & Feed-In tariffs too.
Why is it potentially cheaper? Because the supplier passes all of the risk of a volatile market over to the customer.
From the supply side it isn't so easy to turn up or down the production of electricity so when it's scarce it gets expensive & correspondingly cheap when theres a glut. At periods when demand is forecast to be low, some power stations close down for a while if they can't get a viable price for production.
To offer a fixed rate Energy Suppliers like Octopus have to buy forward contracts which cost them a premium. Works much like currency trading.