The trade works because of team need.
Pompey is probably at least as good as Revere. And despite rumours to the contrary, Michael Saunders is still literally alive (he might not be kicking, though).
Storen completes the back end of the bullpen well. Now there are two good RHP in him and Osuna and two good LHP in Cecil and Loup. It bumps someone like Biagini/Schultz/Tepera who are probably replacement level. It also increases the ability to maybe stretch out Sanchez or Osuna, which may or may not be good.
Since Washington matched salaries, arguably the teams gets a bit better without increasing payroll.
Revere is an interesting case study in past WAR vs. expected WAR though. He has been a league average OF consistently and he has two years of control left. In theory, he should be worth more than 1 year of Storen. But WAR is a descriptive stat, not a predictive one. There are legitimate concerns that Revere's skill set has changed a bit since the past descriptive results. He may be more of a poor LF now defensively than a decent CF. He also doesn't have a "robust" skill-set in any sense of the word, having maxed out his physical tools in the past to only achieve ~2 win results.
Obviously Toronto doesn't see Revere as a rock solid 2 win LF going forward.
It will be interesting to see how he does in the next two years. The Nori Aoki situation from this winter kind of tells us that teams don't value corner outfielders like this (no pop, mediocre D) very much.