DI Boxes are clever pieces of technology.
Sitting between an instrument and the PA, they help with level matching, balancing and either active buffering or passive impedance matching/impedance bridging to minimize noise, distortion, and ground loops. In laymans terms, the DI box acts as a middle man between the instrument and the PA mixer to help smooth out the sound and make it better.
Normal uses for the DI boxes are with an electric guitar but can be used with any instrument which only has an unbalanced 1/4" jack output and needs to go into a balanced XLR input to the mixer, which is a higher fidelity sound.
Basic diagram of how a DI box can work. In this case, the Bass guitar goes through the DI into the mixer but also uses an Amplifier as a monitor speaker, also from the DI box. This allows the player to hear how they sound.
You can plug the instrument directly in to the PA system without a DI box but the quality of sound you're able to produce will suffer for it.
Because they're relatively inexpensive, they can make a great addition to the performing musicians catalogue both on stage and in the studio. The Samson SDIR is a basic one in/one out DI unit, but the Samson SDIRPLUS allows for 2 inputs or for the splitting of one input into stereo.
The SDIR and SDIRPLUS
So if you're looking to improve your sound on a budget or expand your home recording studio, a DI box is well worth considering. Treat yourself today!