John Gill Commentary 1 Samuel 6:7

"Now therefore take and prepare you a new cart, and two milch kine, on which there hath come no yoke; and tie the kine to the cart, and bring their calves home from them;" — 1 Samuel 6:7 (ASV)

Now therefore make a new cart
For there were no Levites, nor priests of the Lord to carry it upon their shoulders, as it was accustomed to be when carried, and therefore they ordered a cart to be made; and they might know the Levites were allowed wagons to carry some of their sacred things on, (Numbers 7:1–8) and a new one for the honour of the ark, as David afterwards did, (2 Samuel 6:3)

and take two milch cows, on which there has come no yoke ;
which also might be designed for the honour of the ark; but there was a further view in it, at least in the providence of God; since two such creatures, who had young, would be apt, if left to themselves, as these were, to return home to them, and not to proceed on a journey; and being unaccustomed to a yoke, would draw one way, and another another, in different ways; and not go on in a direct road, as such that are used to the yoke do:

and tie the cows to the cart ;
in order to draw it:

and bring their calves home from them ;
that they might not cry after them, which would cause them to turn back.