Rigidity, more shock proof, less likely to lose zero?
Yes, they bridge the weakest part of the receiver and help keep the rings in alignment durring recoil. Any misalignment will induce stress into the scope tube which can cause a number of problems. Also the truer the alignment the easier it is for all things to stay tight and where they start out. Rails are worth every penny in my in my book and I have seen the resuts of two piece bases being out of alignment with each other. I bed and check ring alignment on all the bases I install including rails, before a scope is ever bolted in them.
Well said Broz, & hopefully I can add to that. I also believe that the term "you get what you pay for" heartily applies to pic rails. I believe solid manufacturing practices on quality components (steel, Ti etc) lead to a precision, higher quality product.
I believe a low quality pic rail can/will harm accuracy as much as a good rail can improve consistency. When a poorly built low tolerance product is torqued down, I believe you can induce unwanted stress in places it shouldn't be.
I'm not a real big fan of two piece bases, too many variables for me. Some people bed their bases, I prefer to lap them if possible.
Both Broz and Outlaw6.0 make good points and to add to that the local Smith I use also beds the rails. I have not stood right there and watched him but he has explained the process to me and when done properly induces no stress into the rail when torqued to the receiver. This process requires that the rail be as close to perfect as possible which means it's either a Seekins or one from Near Mfg. During the process he jigs the rifle in his mill and checks the rail as he beds it with a dial indicator in all directions confirming that it is absolutely flat.
With a quality rail bedded properly to the receiver and using quality rings there is no need to lap the rings in my experience but always worth checking.
There is something about the outside of a horse that is good for the inside of a man. Sir Winston Churchill.
Everything that is really great and inspiring is created by the individual who can labor in freedom. Einstein
I'm running a 20moa MK4 base on my Lazz... it's non adjustable....correct ring sizing is very necessary of course. A stock pack should easily make up the difference if they do get too high. They're cheap & you can find US made pack (Triad Tactical).