If you have, say, a 2011-vintage tower, you can do quite a bit to upgrade it: add more RAM (4GB to 12GB or 16GB); add an SSD to replace the HD; add or upgrade the graphics card; add ports via an expansion card etc.

Each upgrade can make a significant difference, and you don't necessarily waste anything, unless you change the graphics card. (If you go from 4GB to 16GB, you waste 2*2GB sodimms.)

Seems to me this was also true of earlier eras, too, only with smaller numbers.