Why was the Yak-9U covered with Bakelite?
The Bakelite finish was for the wooden parts/variants.

Bakelite back then was a novelty, its usage in airplanes strengthened the wood, made it moisture resistant, and offered a smooth aerodynamic light-weight finish.

It was also highly flammable:


  The LaGG-1 was built with highly flammable delta-drevesina (delta wood), plywood sheathing, and bakelite ply, and burned so easily that the pilots sarcastically joked that LaGG in Russian stood for Lakirovannyi Garantirovannyi Grob (varnished guaranteed coffin).


The LaGG-3 article on Wikipedia offers more details on this method of construction:


  [The] airframe was almost completely made of timber, with the so-called delta-lumber (a wood-plastic composite composed of very thin, 0.35 to 0.55 mm, wood veneer and phenol formaldehyde resin [Bakelite], baked at high temperature and pressure) used for the crucial parts. This novel construction material had tensile strength comparable to that of non-hardened aluminum alloys.




Sources: MOISTURE RESISTANT FINISHES FOR AIRPLANE WOODS (NACA, 1920), and Russian Aviation and Air Power in the Twentieth Century By John Greenwood.