Hey Piotr_L,
do you think the following answers your question? Or gives a clue at lease? Lori explained the L2 VS use case as following.
Layer 2 Forwarding Virtual Service (Bridge) For situations where a proxy should be used to bridge two different Ethernet collision domains, a layer 2 forwarding virtual service an be used. It can be provisioned to be an opaque, semi-opaque, or transparent bridge. Bridging two Ethernet domains is like an old timey water brigade. One guy fills a bucket of water (the client) and hands it to the next guy (the proxy) who hands it to the destination (the server/service) where it's thrown on the fire. The guy in the middle (the proxy) just bridges the gap (you're thinking what I'm thinking - that's where the term came from, right?) between the two Ethernet domains (networks).
Article:
https://devcentral.f5.com/articles/back-to-basics-the-many-modes-of-proxies
I hope this helps.
Cheers!
Darshan