God is Truth. God is all important. Therefore, Truth is all important.
It is true that some aspects of the truth do not have much impact on what we do, but that does not make them unimportant. Even some aspects of what is certainly known to be true don’t affect how I act. (For example, the fact that St. Constantine is canonized doesn’t much change what I do, as I don’t have nor feel the inclination to have a devotion to this particular saint.)
However, I think you’ll find that knowing the truth can make it easier to follow the Truth, and even those aspects which do not directly influence our actions can help raise our understanding of the truth and grow closer to it.
Knowledge that a little known saint is canonized informs me that this person has an example worth following, and in developing a devotion to this saint and in following his example, I could grow closer to him and through this closer to God. So the knowledge that St. Constantine is saint does not affect me at this moment, but should I develop a devotion to him, I would be enriched. Therefore the knowledge has a use whether I am using it or not.
Likewise for the more important truths, the unthinking Catholic who goes to Church on Sundays, walks down the line, and receives the host may do this whether or not he believes transubstantiation (though he should not if he does not). To the person who disbelieves or does not know about transubstantiation, communion may well be a sign of participation and union with fellow believers. And that’s good, so far as it goes.
But to the person who knows what transubstantiation is, and knows that it happens, he sees just what God is doing for us, that He is using His very Self to nourish us, that He loves us so much to sacrifice Himself for us and feed us on His very Body and Blood, Soul and Divinity.
This is a much richer thing. The knowledge of this makes the sacrament of the Eucharist much more meaningful, and the more this is understood (complete understanding is not possible) the greater we experience God’s love during the exact same action of walking up the isle to receive. Focusing on what God is doing makes the magnitude more apparent.
And so the Truth, even if our physical outward actions aren’t immediately affected, can enlighten us, show us the meaning in every act and make God’s love more obvious to us, more apparent, easier to grasp, and easier to accept.
Again, different truths have different levels of importance, and different people will be affected by the intellectual knowledge of these truths in different ways and to different degrees. I, for example, approach the faith primarily intellectually, with all of the relationship and communion aspects flowing from the intellectual approach just because that’s how I am. I would almost, but quiet, use a bumper sticker that says “Religious, but not Spiritual” because I don’t experience spirituality in the “warm and fuzzy” way that many people do unless it is accessed intellectually (and even then it’s more of an intellectual sense of awe and wonder rather than a feeling of belonging or any such thing).
Your mileage may vary - different things may affect you to different degrees. But I would be extremely surprised if you could not trace your sense of God, your spirituality, at least in some degree to the truths that you do and do not accept.
Why?
