With its Mega Evolution Steelix becomes an offensive threat, whereas before it was just a sitting duck with high defenses. Steel/Fire is a great defensive typing, resisting many common attacking types such as Flying, Ice and Fairy, while only having weaknesses to Fighting, Water and an unfortunate quadruple Ground weakness. It now has a STAB Fire-type move in Stone Edge that becomes very powerful with the boost from Foundry, but the main reason to use Mega Steelix would be to abuse the ability to lay Fire-type Stealth Rocks. With this fiery entry hazard Pokémon like Forretress, Ferrothorn, Genesect, and Scizor all lose 50% of their health on the switch-in while Aegislash, Venusaur and many others lose 25%. Additionally, it is possible to set up both Fire-type and Rock-type Stealth Rocks simultaneously.
With all that said, Mega Steelix is definitely not without its flaws. It really needed a Special Defense increase, but instead it actually decreased by 10. However, the fact that most Ground and Fighting moves are physical helps, which means it only have to watch out for Water moves especially. It also preferred to have a lower speed to bring its best Steel move Gyro Ball to its full potential. It is an easy kill for most special attackers, and while it does have decent Attack, it's just not enough to bring down some of the physical walls such as Hippowdon.
Bottom Line: Mega Steelix is a very viable Mega Pokémon with its ability, and can be a great addition to the team if you're looking for a partner to Dragon- or Fairy-type Pokémon.