Why would I ever want to rasterise text? What is the purpose of this operation?
There are many reasons why you would want to rasterize text. I agree that in general one does not want to do it if it can be avoided. Reasons for rasterization:
You dont trust the end users application to do it correctly or you are having some issues with your own/a known rasterization engine. In these cases you can gain benefits. This can be a valid strategy when sending things to print, or when dealing with computer presentable content/video.
You need to ensure uniform reproduction on different operating systems. Different systems deal with fonts differently and this may cause some problems on embedded devices like phones or Windows vs osX.
Target device is unable to do the rasterization.
You want to share a preview of a document and not share your assets. Rasterization nicely sidesteps the issue.
Enabling basic users to use your work, such as embedding it in a Word document. While there are better options in many cases the average user is not always able to use this approach. Using vectors in word is sometimes a insurmountable problem.
You want to apply some effect to the font. Effects are in general easier to build in a discrete setting and many effects may be impossible to do in full form.