Divine attributes, like human characteristics, can be vague, abstract concepts. If, for example, I say that I love my wife, but spend no time with her, ignore her desires and wishes, reject her requests for my time, criticize her at every opportunity, and pursue relationships with others, she has every right to suspect that my profession of love is empty, meaningless, and worthless. On the other hand, if I make time to be with her and give her my undivided attention, praise her for her incredible giftedness, and forsake all others, as I promised on our wedding day, she will likely think that my profession of love is authentic. My love for her will never be perfect or complete, but it is genuine and is evidenced by what I do.
John declares that Jesus, the incarnate Word of God, is full of grace and truth. Elsewhere Jesus claimed to be the Truth (John 14:6) and that He demonstrated that He was the Truth both by what He said and did (John 10:37–38). Jesus never claimed to be grace; He never used the word. But He incarnated grace—He lived it so consistently that people who recognized their need flocked to Him. And they found in Him a gentle, loving, compassionate, merciful help in their time of need. They walked away changed by their interaction with God in flesh. He was perfect and complete grace and truth.
The God of holiness so wants to be known that He came to us, took on flesh, and lived among us so that we could know what grace and truth looks like. Not grace without truth, which would be false. Nor truth without grace, which would be merely a concept. And not partly grace and partly truth. But full of grace and truth. In flesh. For us. So that we could know God. And for our salvation.