What you may not realize is that the atk/def of evolved card is affected by the atk/def of the Evolver card. The stats of your newly evolved card are the default stats for that card at the grade PLUS 5% of the stats from the Evolver card. That might not seem like a lot, but it can add up.
Take a look at these examples. The one of the left is a grade 2 card as evolve card and a grade 1 as evolver card. The ending stats are 3780/3345. The one on the left is a grade 1 card as evolve card and a grade 2 card as evolver. Its ending stats are 4000/3539.
So our formula is:
Default stats for Grade 3 Kujata Tamer Celeste + 5% stats of Evolver card
That means our example on the left is 3650/3230 + 130/115 and the example on our right is 3650/3230 + 350/309.
The 5% additional atk/def you get from the evolver card can be increased by enhancing, evolving, and even adding materials to the evolver card. The materials themselves will disappear (only materials in the evolve card will carry over), but if you have some extra you weren't going to use, why not add them and make your final card just a little bit better?