Some people think Word can only handle four versions of a font family – regular, bold, italic and bold italic.
Designers want more subtlety in their type than most Microsoft Office users. Designers, who typically use Adobe InDesign, will think in terms of all the different weights or cuts. For example Helvetica goes all the way from 25 (extra light), 35 (light), 45 etc to 85 (extra bold).
When you buy a non- Windows standard font you choose which weights you want. However they are often set up so that they work in a more familiar way in Office – pick the font from a list and then click the relevant buttons to select the bold, italic and both italic and bold. That gives you the four cuts of font. Pretty sensible for 95% of users – pick a font, make it bold. They don’t want to have to go back to the font listing every time they want to change Helvetica (say 35 light) to make it italic (Helvetica 36 light italic).
It is the font foundry not Microsoft that influences the behaviour of the fonts. What the font foundries tend to do is to have standard pairings in the font – so it is likely if you bought three weights that the light is paired with the medium or the bold so you would only see two weights or cuts in your font list. Whereas when you look in the font folder in the control panel you should see all three weights. So if say 35 and 65 were paired you would see Helvetica 35 light and Helvetica 45 Roman in your font listing and NOT see Helvetica 65. But when you highlight some text in 35 light and then click the bold button, behind the scenes Office will apply 65 cut. The user will see Helvetica 35 light and the bold button turned on. For a font that is not paired, the 45 in this example, when you click the bold button Office thickens the type slightly and that’s it.
The difference is more obvious with the difference between a regular and italic serif font (as opposed to a sans serif font). If there is no paired italic cut of a font weight Office will just lean the text to the right whereas a properly cut italic tends to be quite different. If you want to do a test look at what happens to a lower case ‘g’ in Georgia font when switched between regular and italic.
Finally you can ask the better font suppliers to add / remove / change the font pairing to suit your needs
I would suggest for most users the subtleties offered by different weights often goes over their heads. Where companies have an internal marketing/design department they may need more fonts. It all comes down to setting things up so that users can use Word and Office without thinking too hard and so that their documents are in line with the company brand.