Back in 1990, I read several classified documents about the (then) Soviet space program and their satellites. They reused the original Vostok capsule for a variety of other satellite missions. Some of these may have been unclassified, but until I find an unclassified source, I can't talk about them. Here's one - the Bion biology research satellite was derived from a reconnaissance satellite.

Also, instead of developing expensive electronics that can operate in a vacuum (air is an insulator, so high voltage electronics in a vacuum has to be designed to prevent arcing and shorting), they'd use off the shelf components in a pressurized vessel. Sure, it was heavier but vastly less expensive to develop and produce.