Why are they interested in the Sahel? What makes the area so important to western powers?
There are different reasons. Some real, some merely stated (without being real), some believed to be real but not really correct.
In Mali, an AQIM offshoot of Al Quaeda was trying to grab territory. After 2001 and Afghanistan, Western powers consider that a Bad Thing, when an Islamist organization (never mind Al Quaeda) gains control of a territory.
Local geopolitical - your influence
You help a government, you get a local friend. Plus, you are seen as more powerful/influential in the community of nations.
Local geopolitical - regional balance of power
You help a power that opposes whoever you don't want to dominate the region.
I need to dig up Sahel specific examples, but see US military aid to Iran during Iran-Iraq war. Or to Saudis today.
There are large interventionist fractions on both the right and left wings of political spectrum in Western countries. They have varying reasons to want military aid (some more valid than others) in different circumstances, but they all exert political push on the government
One side of the conflict is seen as causing a (disproportionately) worse for humanitarian situation if they win. See Al Assad. Or Mali again.
Alternatively, the conflict itself is seen as causing too much humanitarian issues (well DUH) and military aid is given to shorten it.
You can't provide humanitarian aid to a conflict zone. It's dangerous to aid providers, AND the aid ends up being taken by the most ruthless person around, who most of the time isn't who you're trying to help.
If you help one side win a war via military aid, you help your own weapons industry obtain a market.
Bribery for good behavior.
See US military aid to Egypt since Camp David agreements.
Most of the above aren't really specific to Sahel, Sahel is merely tons in the news due to AQIM strength and involvment and the overall geopolitical situation in the area.
As far as why Sahel:
New markets. Whoever exerts the most influence long term, will have those markets more open to them.
Resources. That's the main reason China and EU are interested in Africa in the first place.
It's an area with weak governments, and reasonably weak regional infrastructure; coupled with tons of conflicts (tribal etc...).
As far as internal politcal pressure, some of those areas were former colonies. Populace in ex-colonial powers feels "responsible" for helping them (I will refrain from sarcastically reminding them of "white man's burden" idea they claim to loathe in public).