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Especially with the longer clubs, it is suggested that your spine angle slightly away from the ball (not straight up and down).

Sometimes the suggestion is just for "better players".

Why do people feel this spinal tilt is helpful/ useful ?

State your opinion !

If I stood straight up, using an interlock grip, my club would have to be about 7 feet long before the clubhead would rest on the ground.

I was talking about spine tilt not spine angle.

OK, I thought your inquisition seemed odd. Sorry I mis-understood

The theory was to help players with aggresive tempos to keep behind the ball, and keep the path from the inside.

I don't like teaching it, because I think it is an un-natural body position which is something you will subconsciously fight against during the swing.

A solid stance and good athletic posture, is better for the body.

i tilt just slightly since i have a tendency to get a little steep with the driver. other than the woods, i don't tilt.

It is only meant for two-plane swingers.

In his book "The Plane Truth for Golfers", Jim Hardy recommends a slight spine tilt for two-planers to create more width due to the steeper swing plane of the two-plane swinger. However, he clearly states that one-plane swingers should NEVER tilt their spine as this would create too much unnecessary width. It has nothing to do with the better player.

As a two-planer, I tilt my spine about 10 degrees to the right (or behind the ball). For me, getting behind the ball somewhat helps considerably. I do this for all shots, from PW to driver.

I hope this was what you were referring to.

Since your trail hand is below your lead hand when you take your grip, your trail shoulder, and thus your spine, must tilt slightly. If you stand with your shoulders level, you won't be able to place your trail hand on the club, at least not in a good setup position. Try it.
Jerry

Spinal tilt, from what I understand, is nothing more than a modern renaming of the age old adage to "get behind the ball", and the reason is ergonomically understandable.
Consider the base of the spine and the hips. As the hips coil during the backswing, the base of the spine, as viewed from the front, moves slightly away from the target, about an inch and a half on average. If the top of the spine does nothing to accomodate this movement such as presetting the head ala Nicklaus, or allowing the head to move away from the target with the lower spine, the spine will be tilted toward the target at the top, which is a reverse pivot.

this is a dead thread from two years ago

Gotta tip our hats to the spammers for bringing it back to life...

Still valid, tho...even though Jerry (thinkin2) gave the real answer.

Shade

In order to get the right hand below left at address you can either open the shoulders or tilt the spine away from target. The latter has many desirable effects, not least of which is that it helps key in the right hip pivot and reduces the risk of a hip slide during the backswing ( kicking in the right knee and ensuring the weight builds up on the inside of the right foot “completes the set”). At the top of the backswing it may prove helpful if the head is over the inside of the right thigh and, during the downswing, it may be useful if you keep it there (stay behind the ball). For specialist shots (particularly with the shorter clubs and difficult lies) it can beneficial not to tilt the spine (i.e. exploit open shoulders).

Nail on the head...great answer...

...also helps avoid a reverse pivot or reverse weight shift. Don't overthink it, though - if your right shoulder (for a rightie) is lower than your left, your spine is tilted. Don't put any more thought into it than that.