A slave lets her Master bind and tie her as He sees fit. This is the ultimate displays of trust she can offer Him. To allow herself to be tied so she is completely at His mercy; her trust in Him must be absolute so she does not fear this position. She revels in His dominance of her and welcomes the chance to prove her love and trust in Him. Her willingness to be placed in a totally helpless position demonstrates how she honors Him.
From the Master’s point of view, a slave’s willingness to submit to bondage is a must. By actually binding her, he displays in a real way the complete control He has over His charge. He glorifies in His absolute domination of His slave and in her total trust of His ability to be that for her. For Him to be able to guide and teach His slave, she must have faith and trust in her Master. When HE binds her and she accepts this action, she has proven her faith and trust in Him.
This may be an over simplification of what is occurring between the Master and slave. Sometimes the simplest answer is the best and most correct one too!