An outdoor electrical outlet is great if you need an easy to reach power source within the confines of your garden for regular use.
Although they're not often used in colder climates where the garden is rarely visited, in hotter countries or even hotter parts of countries south as southern America or Australia the gardens are used a lot. Swimming pools and electric barbeques are two things that feature a lot in the gardens of hotter countries. In fact in most hot countries there's also a lack of rain which means that electrical appliances are used far more frequently outside.
With regards to the electric barbeque for example, if it were in a colder country it would only be taken out rarely for use. Seeing as it wouldn't be used that often it's much more acceptable for a long, twisted extension cord to meander its way through the kitchen to the outside. Now looking at a hot country such as Australia where there are far more barbeques, perhaps several times a week, it's not a good long term solution to have an extension cord.
If on the other hand you have an outdoor electrical outlet attached to an outer wall then you can very easily connected to the mains electricity very quickly. Most houses in hotter countries come with them already attached to the wall. If you do want one installed (and I do recommend it), then you have to contact an electrician who can install one for you. The parts to install one aren't expensive, usually just 30 dollars or so, but the actual electrician will cost more.