One reason is to be able to magazine load longer seated bullets than the limit imposed by standard .308 SA mags. The old concept of "short action" is being challenged by some of the new super-long high-BC bullets in the various 6.5 SA calibers, which are impossible to seat properly to load in a standard SA magazine that is limited to about 2.870". They have to be single fed only, which is fine for bench work at the range, and for single feed disciplines, but which eliminates possibility to use them in repeater-type disciplines like PRS matches that must feed from the magazine. Hence the consideration of LA actions for so-called SA cartridges.
I just dropped my Tikka T3X .260 Rem (short action) into an MDT XRS SA Tikka footprint chassis. Tikka T3/T3X in MDT chassis can use MDT's AICS pattern steel magazines with no binder plate, which allows for cartridge max OAL of 2.960" (which is longer than the standard SA 2.870" max mag length). The Tikka's feed ramp overhang is very small, so the no binder plate version works no problem, and is fantastic!
In my Tikka's factory stock with its "M" SA magazine that it came with, I would have to single feed all my longer seated .260 rounds that were longer than standard SA magazine length. That was OK for bench shooting at the range, as I always single feed at the range anyway. I like to seat 136gr Scenars to about 2.912", which is longer than standard SA mag length. This is now no problem for loading into the MDT no binder plate steel AICS pattern mags, and with room to spare (and I can still single feed). So I am now set up for PRS style matches with my Tikka in the XRS (if I can ever get to one, I live about 1000km away from the closest matches!).
2.960" is probably not long enough to seat for the longer heavier Berger 6.5 bullets (e.g. 156gr, 153.5gr, 144gr), but I don't plan on using those in my .260.
I never could find the Tikka "M+" mags (which can handle longer seated bullets, and which I am guessing were made for the 6.5x55 Swed) for sale anywhere (sold out), and I emailed Stoeger Canada for help on this. As usual, Stoeger was no help, would not sell me a M+ mag, would not order me an M+ mag, did squat. (Stoeger don't seem to want to learn about customer service with my Sako and Tikka rifle parts, but I digress....)
The Tikka T3/T3X fans here all know that with the standard .308 size bolt face, it is easily converted between short and long action with the bolt stop swap (Yo Dave sells Tikka SA and LA bolt stops), and a Tikka SA, MA (M+), LA magazine swap in the Tikka factory stock. With an aftermarket stock/chassis, you are stuck with what the bottom metal allows for mags.
Anyways, the Tikka T3/T3X action is a very versatile system for shooting SA, MA, and LA standard bolt face cartridges.