Let me start with the the term 'bridge'. This would be an example of two cycles with a 'bridge' between them:
Week 1-10: Dbol, Test, Deca
Week 11-15: Test only at a smaller dose (this is the bridge)
Week 11-20: Test, Tren, Winny
Now we have kids saying they are 'bridging' pro-hormones like:
Week 1-4: SD
Week 3-7: Epi
THIS IS NOT A BRIDGE. IT IS USING TWO STEROIDS CONSECUTIVELY DURING THE SAME CYCLE.
Then what is a blast & cruise you ask? It's basically multiple cycles 'bridged' together with a small dose of test and sometimes a low dose of another anabolic like deca, eq, or mast, or maybe even some peptides.
So to answer your question, it is common practice because in theory it makes sense to a lot of people, and in reality you will still make great gains. 25 years ago it 'made sense' to not use testosterone when there were much more anabolic and less androgenic substances out there. (sounds stupid now huh?) You can still make huge gains with tren only. Test is not needed if your goal is just to get huge. So who's right? The guy who's 250lbs of ripped muscle who only uses tren? Or the guy who's 250 pounds of ripped muscle that uses test with all of his cycles?
My point is this, whether or not you are overlapping, or just running one compound then switching to another, you will still make good gains. YOU CAN DO IT EITHER WAY. It's just that running a cycle for an extra 25% longer will yield more gains, period. So if you start with X amount of compound 1 and X amount of compound 2, it is most effective to string them together for the longest cycle you can handle. The less you overlap, the longer you can run it. The longer you run it, the more gains you make. The 'transition' will be 'smooth' regardless of an overlap.