With car rental I fully agree. Especially as with most deals you pay nothing up front.
Often when I have rebooked, it has not been the price, but rather that I have changed where I am driving, length etc.
In addition booking early normally achieves a wider choice of vehicles to select from. Moreso when you want to return to a different location.
And with returning to different locations I have normally found the one-way drop-off fees to be less or zero.
Accommodation paid, or part paid in advance, often gets much better deal and so when I do not mind being locked in I will for many trips book it in early and moreso for smaller viillages/cities where the choices are more limited. I often like smaller quirky accommodations and so booking early gains the place I want in the location, location, location I want.