In Visitor : supporting documents guide GOV.UK has five pages of documents that are either obligatory or recommended. The very first of the recommended is Previous travel documents/passports, which show previous travel.
Maybe your old passport contains no such evidence, or you will never seek to enter UK, but otherwise it may make your entry smoother.
Personally, I have found other people's old passports of interest for historical and genealogical purposes. For example, some of my mother's passports mention a middle name she never liked - and managed to get removed from a birth certificate. (I have since found another birth certificate that does show that middle name, together with some slightly different other details but have the link to prove the two certificates are for the one person.)
The revenue stamps may have some value to collectors (though these are not used nowadays, they did used to be).
One of the visas gave me the name I was seeking of the vessel that transported me across the Atlantic (at an age when I was oblivious to such things).
In summary, even if no use to you at present, they may be of interest to others, eventually.