It is important to hear the Holy Spirit directly because, the letter kills, while the spirit gives life. Trusting the Lord takes a lot of courage, and can be risky. Risks are what scare us away from trusting God.
In other to trust God, you have to take away backup plans and alternative solutions. Without help from the Spirit, the fear of risks involved in trusting Her would overwhelm and trouble you.
On the long run, trusting God will produce bliss, but on the short term, it produces undesirable experiences. When you avoid the troubles that come as a result of trusting God, you become stagnant in your faith. The Holy Spirit is with us to give us the power to embrace the risks and let go of the backups