What's the reason for stroke spasticity drugs?

This is only my theory, so be sure to ask your doctor for his/her explanation.
There is a great hope that by giving general muscle relaxants the antagonist muscle has enough power to overcome the spastic one. This also makes the assumption that the antagonist muscle control is only partly damaged and not dead. Botox or phenol basically paralyze one muscle while at the same time hoping that the antagonist is only damaged. This is a completely trial and error guess by your doctor because they have never come up with a specific damage diagnosis and thus are not able to compare cases against each other. They are completely flying blind.
This one is interesting for what it eventually says.
http://www.drugs.com/newdrugs/fda-approves-movement-disorder-cns-therapeutics-gablofen-baclofen-severe-spasticity-2435.html
This is just another way to deliver baclofen, similar to the baclofen pump. This line from the article is key; and the need for intrathecal baclofen therapy is life long since it does not cure the cause of spasticity. Remember none of the drugs for spasticity,even botox, cures spasticity.
Ask your doctor about side effects.
Abrupt discontinuation of intrathecal baclofen, regardless of the cause, has resulted in sequelae that include high fever, altered mental status, exaggerated rebound spasticity, and muscle rigidity, that in rare cases has advanced to rhabdomyolysis, multiple organ-system failure and death.
The most common adverse reactions in patients with spasticity of spinal origin were somnolence, dizziness, nausea, hypotension, headache, convulsions and hypotonia. The most common adverse reactions in patients with spasticity of cerebral origin were agitation, constipation, somnolence, leukocytosis, chills, urinary retention and hypotonia.

Because there is no decent understanding of spasticity or any known cures there is the hope/placebo that the drugs given will help.