The credobaptist (one who believes in believer’s baptism only) robs himself of the chief benefit of baptism.
They usually hold their position on the grounds that baptism must be done as a response to conversion- only converted people should be baptized. Therefore, baptism becomes mainly associated with their own decision for Christ. With this focus on baptism, they become robbed of one of the primary benefits of baptism, the testimony of what Christ has done for us. Baptism is a sign pointing me to the forgiveness of sins and the gift of the Holy Spirit. It is a testimony to me of all that God has done for me. In Romans 6 Paul calls them to remember the fact of their baptism when struggling with temptation- in baptism, the believer is promised new life in the Spirit of God, a life which is inconsistent with sin.
But when the credobaptist considers his baptism, he is reminded of what he has done. He is pointed to his own faith decision. He is focused on what he brings to the table. It is not uncommon for credobaptists to become baptized multiple times, unsurprisingly given their belief regarding baptism- if baptism is something I do, then it will always be inadequate to my needs and I will repeat it over and over with the hope of generating a positive enough experience or earning God’s favor enough to conquer sin. But if baptism points me to what Christ has done for me, then it need not be repeated- indeed must not be repeated. Christ’s death is sufficient; the Spirit is poured out on me like the waters of baptism are poured on me, and in Christ I have everything I need. We can remember that baptism, remember that blessing which we have received from Christ forever, whenever we hunger and thirst after the things of God.
Of course the Credobaptist has access to this same benefit from his own baptism. He need only repent of his vain idea that baptism points him to his own decision for Christ, and instead recognize that baptism was given to point us to His decision for us.