The tweaking DM

I was thinking the other day about how DMing seems to involve (at least) two quite different sets of skills. On the one hand are skills of imagination, expression and improvisation -- the "art" of DMing, if you will. On the other hand are skills of systems, rules and judgement -- the "science" of DMing.

Nowadays, my "sciencey" side tends to focus on the tweaking and re-working of core rules and house rules. The new classes I've been posting recently are a perfect example of the expression of that urge. I see something I'm not so keen on in the core rules, play around with tweaking it, and finally come up with my own variant (house rule). Or sometimes I come up with a new idea that just doesn't fit neatly in any of the existing mechanics.

Whatever the case, "tinkering" is, I believe, the phrase.

I've noticed that in my Labyrinth Lord campaign I've gone through a few eras in regard to the rules:

1. We started off with just the basic LL rules (i.e. not using the AEC) + quite a few house rules tweaking various bits & pieces.
2. Gradually stuff from the AEC was introduced. Separate race & class. Attribute modifiers due to race and age. Multi-classing. Secondary skills. (This phase was, interestingly, completely player-driven. They basically saw cool stuff in the AEC and asked if they could use it. I said yes.)
3. After a while I began to pare back the house rules, seeing that some of them made no impact as they were never used, and that some of them were just tweaks for the sake of tweaking, and didn't really add anything to the game. The idea of playing as closely as possible to the core rules became very appealing, partly due to the fact that several of the players also have the rule books, so it makes sense to stick to what everyone has a printed copy of.
4. Now I'm in a phase again where I'm getting itchy feet and creating new classes all over the place and thinking about radically different systems of clerical magic.

So, a real oscillation between periods of the rules being very static and periods of wild splurges of modification / addition can be seen.

Firstly: is it just me? I get the feeling a lot of DMs do this. Do any / many DMs not do this?

The main point I want to raise here though is: what is this all about?

I don't really have an answer to that question at the moment. Maybe it's just fun to play in the sub-world of game-mechanical creation? Partly, for sure -- I love that.

I think it's also connected to the process of imagining a world though. What kind of characters, monsters, magic and adventures exist in that world, and how do the rules of the game support / restrict that?

Anyone else out there got any thoughts on this matter? Either from the DM's or the player's perspective.