If you look at a map of Mars, you see Mons Olympus, and three other giant volcanoes. Those three are in basically a straight line. They are also tilted with respect to the equator in such a way as to evoke the Belt of Orion.
Also of interest to me is how Mars, being smaller than Earth, has some of the most dramatic terrain in the solar system, even counting moons! Valles Marineris, Hellas Basin, the Polar regions, the stark north-south dichotomy. It's weird...
Coincidence? I think not, and I want to know what the buzz is on the topic of Mars.
Volcanoes form above underground magma hotspots. Hotspots which move over time, so a single one can form several volcanoes, usually in a straight line. Most likely, the volcanoes you refer to - I'm guessing Ascraeus Mons, Pavonis Mons and Arsia Mons - all erupted at different stages of a hotspot that has since been extinguished. For comparison, the Hawaiian islands of Kauai, Oahu, Maui and, well Hawaii, form a similar ridge, again in a straight line.
As for why Mars is such a hot topic - it has all the resources for a potential self-sustaining human colony. Its atmosphere is thin, but rich in CO2, which can be used for producing breathable air and growing of plants. It also has strong enough winds to produce power with only a few wind farms, compared to the massive solar panel fields you'd need otherwise. Its surface gravity is close to that of Earth, so a long-term stay wouldn't have as many adverse effects as on a Moon colony or a space station. Its temperature is within survivable limits, compared to the living hell of Venus or Mercury, or the absolute zero just about everywhere else.
All in all, it makes for a rather juicy colonization target, even with the currently six-month trip you'd need to get there.