More Land Area
Earth's human population is booming. Cities are growing larger and more resources---most notably, food---are needed to supplement the expanding population. Although today's technology doesn't provide a cost-effective or practical solution for colonizing Mars, one of the benefits---assuming technology allowed it---would be land area. Mars, unlike the moon, is full of rich elements such as nitrogen, oxygen and carbon dioxide. The planet also contains frozen ice caps. As ice can subsequently provide water, the ice caps might be a plentiful source of this critical necessity of human life.
Earth Similarity
People evolved on Earth; similarly, if they are going to survive somewhere else, it needs to be among the relatively same conditions. Mars isn't an Earth clone, but it's about as close as we're going to get to it in our solar system. According to RedPlanet.com, at one point in history, Mars was extremely similar to Earth, with flowing water and volcanic activity, as can be deduced from its surface structures today. The length of Mars' days roughly match that of Earth's. Although the gravity is less, it is still more than that of the moon. The temperature fluctuates much more severely on Mars than on Earth, but it is survivable under enclosed conditions, averaging close to -85 degrees.
Proximity
Although it's not a two-day trip, Mars is still relatively close to Earth compared to most other planets. Depending on the time of year, Mars can be different distances from the Earth. According to Universe Today, the closest distance will range around 34 million miles, while the longest distance will be around 250 million miles. Depending upon this distance, the average trip to Mars will take a couple of years with current technology.
Exploration
One of the main reasons humans should colonize Mars is for the sake of exploration. The reason humans have colonized anywhere is due to the fundamental migration that has been a part of mankind's behavior for thousands of years. Exploration took Homo sapiens out of Africa; took Europeans to the Americas; and eventually took man to the moon. Mars will remain a challenging next step.