Besides, it's just practice with それに〜

Grammar Explanation: In addition to with それに〜

Urushi plays three shogi games at once against the other members of the shogi club. After she wins all three, she worries she has hurt their confidence right before the upcoming tournament.

うるし:: 「いやいや！！みんなよくやってたぞ」; “No, no! You guys did great!”; 「それにただの練習(れんしゅう)だ」; “Besides, it's just practice.”

Key Points

1それに adds another reassuring reason

Urushi first praises the club members’ play, then uses それに to add that the loss does not count for much because it was only practice.

2ただの練習(れんしゅう)だ softens the loss

ただの means “nothing more than” or “just”, downplaying the match as practice.
The plain だ keeps the reassurance direct and casual.