Because we are commanded to be baptized. It is as simple as that.
How is it important?
It clears our consciences that we have followed God's command. Those who don't wish to exercise their faith in baptism should examine their faith.
and finally, if it is important, why would the Holy Spirit allow no one to correctly understand it for 16 centuries?
Baptism saves you as an appeal to God for a clear conscience (1 Pet 3:21). It's the act of obedience to God's commandment, however we understand it. This act of obedience is for our benefit. That isn't to say that those who are not baptized will not be saved. It simply shows evidence that if we have a desire to be baptized (whatever form we believe to be correct) than it is something that we can look back at in our lives as a starting point.
Presbyterians who baptized children under the covenant view, claiming God's promise that one day He will lead them to Christ, can look back on that promise. Baptists who believe in immersion for renewing of the mind and soul can look back on the day they were baptized. I've come to believe that it doesn't matter whatever interpretation is correct. It all serves the same purpose. It is given to us as a sign of Christ death, burial and resurrection that we can cling to.
I might add that communion (or the Eucharist) is just the opposite. Unlike baptism where we look back, communion looks forward proclaiming the death and resurrection of our Lord Jesus until the day He returns in His glory.
Christians have taught and believed that the Sacrament of Baptism is how we receive regeneration for 2,000 years.
since the 16th century, some sects rose up to teach regeneration another way. they had a problem, what to do with this pesky “baptism” command.
their answer was to turn baptism into a matter of “obedience”. why? just because.
so much confusion reigns that it doesn’t matter what you think about it, Baptist or Presbyterian.
as for why no one understood this for 16 centuries, it doesn’t matter. it’s only to clear your conscience, so who cares?
makes sense to me.