It is based on trade considerations.
Liberal nations usually enjoy pointing out how disorderly African countries while forgetting to mention how reliable they are to supply liberal nations with resources.
On the other hand, Russia, while being orderly, has proven an unreliable trade partner: russians did not stay away from turning off taps when their best interests were compromized. And Russia has the military to back the decision up.
As Europe was cut down, the expectation is they look up for reliable trade partners. Which can be found in Western Africa.
The place has gas and no matter coups, change in governments, the gas will flow out. Supply of Europe is secured.
So building pipelines from gas suppliers to lets say Algeria would ensure reliability.
Here come Russian trade interests: by getting a military force on the ground, Russia ensures they will be part of any project to draw pipelines from western african gas fields to Europe.
While Russia may not be able to get military forces in countries that will produce gas, they have already in countries that would be crossed by pipelines.
No Russia associated to a project, no security for pipelines.
Whoever means trade means trade routes.
Whoever means control over trade means control over trade routes.
Russian presence in Western Africa has nothing to do with the mercenary group, nothing to do with Putin, all to do with trade interests.
Europe is dependent on Russia gas, a dependency they do not control the terms fully contrary to a dependency on African gas.
By getting boots on the ground, Russia ensures the integrity of their trade terms.
Europe will not be able to escape Russia by crushing African countries.