Why so many fonts?

If you look in, say, the BaKoMa font package you will see at least 50 fonts (unless you have the minimal installation). Many of them have the same name but end in a different number. For example, cmr17, cmr12 and cmr6. TEX and LATEX have type styles and fonts are these styles at particular point sizes. For example, cmr is the type style and cmr10 is the cmr style at 10pt. RISC OS has a single font for each type style, which is scaled to the appropriate size. Why does TEX use all these different fonts?

Many people believe that fonts should look slightly different at different font sizes and that scaling isn't enough (although the RISC OS font system does something cleverer than simple scaling). For example, here are cmr17 and cmr6 both shown at 17pt 1:

{\demobig Here is some 17pt text in cmr17}

{\demosmall Here is some 17pt text in cmr6}

As you can see, cmr17, which is designed to be used at 17pt, has thinner characters and appears lighter than cmr6.