There are three levels of engagement with any ruleset. Designer level, Group level, and Individual level. Where certain rules are located makes a big difference in the feel of that ruleset.
Designer level states the rules unequivocally. This is so. That is different. This subsystem is used in these circumstances. The feel is "take it or leave it." Changing rules on the Designer level requires a commitment of anyone wishing to change those rules. Will doing this change affect play in unexpected ways? Will play become unwieldy? Will one thing become too important to the detriment of the game? One must be bold and cautious at the same time. Weigh the expected consequences and institute the change. In the early development of RPGs, GMs were expected to meddle in this area, and the systems were designed with loose tolerances to facilitate these changes - like an AK-47, it would still work even full of mud. This is one of the charms of Old School design. As RPG design developed, developer level design tightened up considerably. Streamlining mechanics forced greater interdependence of components, and tinkering on this level became hazardous to games.
Group level gives groups options. Here are modifiers you can use. Award these points as you see fit. Interpret this broadly. Use common sense. The feel is free and open. Rules on the Group level are designed to be changed, modified, messed with. Group- level rules recognize and deal with the fact that what fits one group may not fit another. As noted above, almost all Designer level rules were also Group level in the beginning. As RPG design developed, Group level rules became a separate distinguishable level. The designer is saying "Here - you can mess with this all you like, and it won't screw anything vital up."
Individual level rules have always been there. You have X points to allocate to Y attributes. Roll XdY and choose the attribute. Choose your profession. Roll or choose from table A. The feel is complete freedom within parameters. The more freedom given on this level, the more wide open the game feels to a player. Constriction of choice at this level is a consequence of the focus of the game. Games more focused on a genre, theme, or story generally restrict player choices more than more general games.
Another tool to play with. Enjoy!