That you may settle yourselves upon their backs and then remember the favor of your Lord when you have settled upon them and say, "Exalted is He who has subjected this to us, and we could not have [otherwise] subdued it.1
Translation: Saheeh International (English)
1 Literally, "made it a companion" or "made it compatible."
He had created for you all that, in the hope that you settle yourselves on the backs of your mounts during your travels and then remember the blessing of your Lord over you in your hearts, in that He has made them subservient to you when you mount them, saying with your tongues: “The being who made these mounts subservient for us is pure and glorified. We control them, whilst we would not have been able to do so if it had not been for His making them subservient to us.”