The conversation about growth rate seems confusing. Let me explain it a little bit. (edit:Oh god I ranted forever why does this always happen)
You've got your base value for the stat, which is the most important; increasing this makes all further bonuses equally larger. Then you have your levelup bonuses, and increases from Patchouli's Library, which in the end work more or less the same, increasing your stat by a small percentage of it's base amount. And finally, you have your equipment, which plants one more big fat bonus on your stats, again from the base amount, independently of the levelup/library bonus. But, tl;dr, there are three ways your stats are raised, and the first (base value) will up the latter two at the same time, so it's big... especially when your BV was low in the first place.
Base value is only increased by the Stat Boost Gems and Stat Boost passive skills (the gems/books of enlightenment require Tome of Reincarnation to reset off the character) and by a few specific pieces of equipment (mostly, if not entirely Main Equips), and a small amount by subclasses... a large amount if you pick Transcendent. Base value boosts can be very powerful, and the most valuable cases are when the stat is important, but the character's Base Value is low... because adding 10 makes a bigger difference to 50 than it does to 100.
You can see base value on the special character info pages where you can see their Battle Points and how many gems you've used on them and stuff. It's also on the wiki, although the way the number is displayed is slightly different (If the game's page says 56, the wiki says 5.6). Gems boost it by 1 (or .1), while Boost skills increase it by 2 (maxing at +10, or you're rinnosuke and get like five times that) so... the difference is pretty small without, say, 10 gems.
It's possible to use base value increasing to build a character for a role they don't seem to be meant for, but in most cases it's best for just making them a little better at something they're already doing; like how a tank!Komachi can really use some investment in her low defense stats since she's already tanking, but her BVs are rock bottom. Or for characters who have good damage, but actually have fairly low base Magic or Attack; you'll hardly notice if you increase the BV of someone who already has really high Attack.
The reality though is you can't do this as much as you'd like. I barely (or didn't) have 10 gems for DEF/MND by the time I even beat the normal-game final boss, although I had more like 20 atk/mag gems and 30 hp/mp. And the Boost skills on not-rinnosuke are generally lower priority than a lot of their other passives or spellcards. And Transcendant isn't even -unlocked- until quite late in the game, along with taking a buttload of skill points to actually use. The main equipments to boost BVs all have significant tradeoffs and non-drastic effects... with the exception of the First Aid Kit (greatly boosts HP base value) which is godly on basically anyone, make your tanks tankier or your squishies suddenly not squishy, those things are priceless and you'll never have enough of them.
So, it's not the godliest tool, but it's there, and it can help some. It might also be worth pointing out that some characters with bad base values might have better effective stats than it looks due to cheap library levels... and vice versa. Late in the game you might be able to get their stats at lv180 for similar prices other people get to lv130 for... or only hit lv100 for that price. (And then, of course, that means that much more/less of a difference if you increase base values)
Side note:In LoT1, the equipment bonus is lumped in the same as the levelup/library bonuses, so late in the game when you have a ton of those the equipment bonus is really low... you mostly wear it for resists/affinities at that point. Equips are a lot more important in LoT2 due to this change, among other reasons.