Why

[rambunctious]

Helping a mate with a couple of jobs, one being the roofing of his pergola, and noticed his pergola posts had what I call structural stirrups but with bolt heads both sides, obviously coach screws not bolt right through.
Pointed it out to him and he decided to change them.
Of course the question must be asked, why have structural stirrups not pergola stirrups, and why use coach screws.
Never ceases to amaze me what DIYers do.
Pics tell it all.

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[ErrolFlynn]

coach screws not bolt right through.

It would be difficult to drill the hole straight and square.

If you take a perfectly straight and true square steel bar and use a bench press to drill a hole there's a good chance the exit hole will be where you expect it to be in relation to the entry hole. Most holes drilled into timber are made with hand-held tools. Sometimes the drill bit seems to have a mind of its own and wanders. It would be next to impossible to make a perfect hole without some kind of jig. The exit hole will not be where you'd like. I suspect the coach screws were used to guarantee neatness in the finished job. Perfectly reasonable. And yes, a bolt all the way through would have been better, but not really possible in my experience. At least, not in a practical way.

[r3nov8or]

There is lots we don't know...
The stirrups could have been what was on hand, or gotten cheap, or were the aesthetic the owner wanted.
If the pergola is free-standing, "high-wind" stirrups (what I call them) may have been chosen for lateral stability.
If the pergola didn't originally have a roof (that's what a pergola is, unroofed ) then uplift forces would not have been a concern and coach screws would be fine.
Installing a roof, sure, now is the time to use through-bolts.

Re getting the holes right, you just drill part/half way as accurately as possible from both sides and the drill bit will be drawn to the exit hole without problem. Done many times...

[rambunctious]

There is lots we don't know...
The stirrups could have been what was on hand, or gotten cheap, or were the aesthetic the owner wanted.
If the pergola is free-standing, "high-wind" stirrups (what I call them) may have been chosen for lateral stability.
If the pergola didn't originally have a roof (that's what a pergola is, unroofed ) then uplift forces would not have been a concern and coach screws would be fine.
Installing a roof, sure, now is the time to use through-bolts.

Re getting the holes right, you just drill part/half way as accurately as possible from both sides and the drill bit will be drawn to the exit hole without problem. Done many times...

Actually had no trouble drilling straight through straight and true.
Guess it depends how you hold your tongue.
14 holse, only troublesome ones were the 2 that broke off in the post.
Had to start with a 6mm bit to drill them out of the way.

[r3nov8or]

Actually had no trouble drilling straight through straight and true.
Guess it depends how you hold your tongue.
....

Yeah, the existing holes on both sides from the coach screws would have helped I reckon. Tongue on top left molar helps too

[Skew ChiDAMN!!]

I think I prefer "I think I can do that myself!" level DIYers to use coach screws, even though it is a less than desirable approach.

Many, many moons ago I had the misfortune to badly slice my ankle open on the rough-cut end of a coach bolt used in an upright. The owner/builder was mortified as he hadn't thought of that possibility. To his credit, I thought the rest of the build was very well done... but I still ended up in emergency.

Coach screws avoid that issue altogether, unless they horribly mangle the heads. (In that case, odds are good that'd be among the least of their issues...)

[Mobyturns]

As long as the fixings comply with the structural & wind loading for each joint type either would be fine.

I think the coach screws vs bolts are the least of the concerns - whats happening below the pavers???

Galv is fine corrosion protection but like all things things it has limits, such as expected service life when exposed to contaminants, or reduced service life due to poor design / installation practices. The neatly cut in pavers look great - however a raised and correctly finished column footing would be a far better solution for either stirrup or post anchor supports..