I think this writing by Thomas Aquinas gives a clear understanding of why we use sacraments in church:
‘Since it is connatural to a human person to acquire knowledge through the senses, and since it is most difficult to arise above sensible things, divine providence has appointed sensible things as a reminder to the human of things divine, so that human intention might the more readily be recalled to divine things, not excluding the one whose mind is not equal to the contemplation of divine things in themselves.... Again, sensible things are employed for human sanctification, in the shape of washing, anointings, meat and drink, and the uttering of sensible words, as signifying to the human that one receives intelligible gifts from an external source, and from God whose name is expressed by sensible words. Moreover a human performs certain sensible actions, not to arouse God, but to arouse oneself to things divine: such as prostrations, genuflexions, raising of the voice and singing. Such things are not done as though God needed them ... but we do them for our own sake, that by them our intention may be fixed on God, and our hearts inflamed.’
Summa contra Gentiles, III 119