The most common reason for needing to increase the size of text on a screen is so that it can be more easily seen and read. Another possible reason is to improve the aesthetics of whatever is on the screen, such as a video or presentation.
The most common reason that people need to enlarge the size of text on a monitor screen is so that it can be more easily read, either from a distance or by people who cannot see well. For example, during a presentation to a large group, people in the back of the room would appreciate larger text that can be read from that distance. There are zoom options on the computer that can be accessed using shortcut keys, in the browser settings or on the computer's control panel settings.
A second reason one may need to increase the size of text on a monitor would be to make whatever he is working on more aesthetically pleasing. What this means is that if someone is working on a presentation or a video, often it looks better if some text is larger and some text is smaller. This can be easily changed within whatever program is being used to create the media.