There are multiple parameters because of the trade of at print speeds.
Slow printing will have less ghosting and a better over all quality, but it takes way more time.
Fast printing will be fast (oh thank you cap. obvious), but you will get stronger ghostings, less good lines over all. Maybe some strange artifacts.
But you don't need a good print quality at parts that are not visable at all, but you need one on the outer shell. Also is the travel mostly just something that you want done as fast as possible but maybe not with the issue, that the whole printer starts shaking.
So you want to set different parameters for different parts.
Next reason: most slicers are used for a large variation of printers, and every printer is a litte bit different, even the same printer type with different addons (like a vanilla Anet A8 vs. a beefed up Anet A8). To be able for the slicer to be used by all these printers it is necessary to be able to set this multiple parameters.
To get deeper on your question about the different infill speeds: If you print a solid infill you will also need to give the printed material more time to cool down, because it is packed tightly and there is no much surface area for getting heat away. If you print these areas to fast you will end up with a part that is too warm and warps and deforms all the time while the next layers were printed on already, inserting even more heat into the part and isolate the warm parts from the outside. You can observe similar behavior when printing overhanging areas with a thick walls. There the part will stay in a semi solid state and deforms all the time.
But how fast you habe to go depends on the cooling system you have on your printer.