There are many reasons why swimming is such a great way to exercise. With the wide availability of indoor pools, swimming shouldn't be considered an activity that can be done only in the summer -- it can be done year-round. While weightlifting and running can be very bad on your joints, swimming is low impact and puts very little stress on your tendons and ligaments. Additionally, most forms of exercise focus exclusively on building strength or improving stamina, but swimming improves both simultaneously. The reason for this is that swimming is a constant full-body workout, which saves you the time it would take to go from machine to machine on a circuit for the same effect, and it also saves you money on paying for expensive workout equipment.
Swimming has built-in benefits for your long-term health that other traditional forms of exercise like running and weightlifting simply do not have. First of all, it's fun! A lot of people have trouble sticking to an exercise regimen because they dread going to the gym or getting out on a track or onto a treadmill and running. When your exercise is more fun, you're more likely to get up and do it, which means more consistency in exercising and better long-term health.
Additionally, swimming makes you much less prone to injury than most other types of exercise. Running can cause painful shin splints, and weightlifting can cause major damage if you push yourself too hard.
Because swimming is a single activity that can give you a total-body workout in a short period of time, you have to consider the advantages of all of the time you'll save by adding more swimming to your exercise plans. Once you have a more compact and effective workout, you will have more time for other areas of your life, or you can decide to use that time to work out even more -- picking up a yoga class that you wouldn't have had time for before, for example.