In a nutshell, here is why. What's worse than striping a shot, hitting the sweet spot, swinging in balance - and watching the ball veer off target and end up in a bunker or water hazard? It's frustrating, and if your clubs aren't custom fitted to you, it could be happening often. What's worse, clubs that don't fit your swing could force you to develop poor swing habits as you manipulate your body, your setup and your swing in order to get the ball to go roughly where you want it. And yet it happens everyday to the majority of golfers! Although buying off the rack can work for some, and in certain cases, a standard set of golf clubs could actually fit someone's swing - for the most part clubs need to be tweaked in order to maximize performance and enhance instead of hinder your golf.
Simply stated, the lie of the golf club is the difference in the angle of the grooves vs. the horizon at impact. If the grooves of the golf club are parallel to horizontal, the lie is ideal. If the grooves are not, then a lie adjustment is necessary. If the toe of the golf club is up in the air vs. the heel at impact, the club is too upright and the grooves will produce hook spin. The opposite is true if the heel is up vs. the toe at impact, the grooves will produce cut spin.
So, if you make a perfect swing and the club is too upright, you will pull it or hook it. If the club is too flat, you will fade it or push it, regardless of how well you strike it.
Obviously there are challenges to hitting the ball well if the clubs are too long or too short for you. Firstly, it can cause some poor swing habits as your contort your body to make decent contact. Secondly, a shaft that is too long will push you away from the ball making the club dynamically too upright, a short shaft making the club too flat. As mentioned above, a lie that is wrong produces poor results.
Flex is important because it affects both ball flight, launch and feel. If your club shafts are too stick, the face can't square and it will lead to low, blocked shots, and will limit your feel. A shaft that is too soft will often cause a pull as the flex of the club shuts the face at impact. It'll also impart significant spin that will cause the ball to up-shoot and die in the air, especially when playing in windy conditions.
With the driver specifically, you really want to control your spin, because poor spin characteristics are a big distance and accuracy killer. Shaft flex, as well as launch and load are extremely important in maximizing distance, both carry and roll.
For the most part the rule on grip size is... if it's too big, your hands won't release properly so you'll push shots and lose distance. If it's too small it can lead to a re-gripping of the club at the top leading to a shut face at impact and shots that miss to the left of the target.
The above points cover merely the basics of club fitting. The takeaway from this post is that if your clubs were not custom fitted to you, there is a very strong chance you are hitting shots that are going sideways and are not the fault of your swing, but the fault of your equipment. And worse, there is a good chance you are developing poor swing habits as you try to compensate for poorly fitted equipment.
Any golf club can be fitted, and all of the major vendors, like Callaway, Taylor Made, Ping and others can produce a custom set of clubs that exactly fit you.
Want more information? Leave some comments and I'll point you in the right direction!