"And that you may attain upon them a need in your breasts," meaning: the camels. And 'need' means: travel.
﴿وَلِتَبْلُغُوا عَلَيْهَا حَاجَة فِي صدوركم﴾ يَعْنِي: الْإِبِل وَالْحَاجة: السّفر
