A guide will help you to get the most out of your trip. They will be able to take you from place to place, so when transport is scarce, or perhaps expensive, your guide will be able to help. They will also know the best spots to take you to, making sure you see the best parts of your destination.
You can often hire a guide when you arrive, which is usually the cheapest option. This is also probably more beneficial for the economy of the country you are visiting, as the guide will be a local person. If you hire a guide from home before you leave, another company will probably be profiting from your decision, rather than the local community.
You could hire a private guide for a day, or you may prefer to go on a guided tour and share a guide with other travellers. This will be cheaper, and a chance to meet new people.