According to Bhishma, Kunti, Gandhari, and Madri had beauty that were fit to expand the kuru kingdom
It hath been heard by me that there are three maidens worthy of being allied to our race. One is the daughter of (Surasena of) the Yadava race; the other is the daughter of Suvala; and the third is the princess of Madra. O son, all these maidens are of course of blue blood. Possessed of beauty and pure blood, they are eminently fit for an alliance with our family.
Boon from Shiva that she would have a hundred sons.
"Vaisampayana continued, 'Soon after Bhishma heard from the Brahmanas that Gandhari, the amiable daughter of Suvala, having worshipped Hara (Siva) had obtained from the deity the boon that she should have a century of sons. Bhishma, the grandfather of the Kurus, having heard this, sent messengers unto the king of Gandhara.
Shakuuni's father hesistated a bit because of Dhritarashtra's blindness but even he accepted the alliance based on the fame of the Kurus.
King Suvala at first hesitated on account of the blindness of the bridegroom, but taking into consideration the blood of the Kurus, their fame and behaviour, he gave his virtuous daughter unto Dhritarashtra and the chaste Gandhari hearing that Dhritarashtra was blind and that her parents had consented to marry her to him, from love and respect for her future husband, blindfolded her own eyes.
Bhishma wanted Pandu to have a second wife and he chose Madri because she was celebrated for her beauty and virtue.
It hath been heard by us that thou hast a sister named Madri celebrated for her beauty and endued with every virtue; I would chose her for Pandu. Thou art, O king, in every respect worthy of an alliance with us, and we also are worthy of thee. Reflecting upon all this, O king of Madra, accept us duly.'
That is the reason why Bhishma even gave Shalya treasure because there has been a custom since his ancestors that the groom's money have to give them money.
The ruler of Madra, thus addressed by Bhishma, replied, 'To my mind, there is none else than one of thy family with whom I can enter into an alliance. But there is a custom in our family observed by our ancestors, which, be it good or bad, I am incapable of transgressing. It is well-known, and therefore is known to thee as well, I doubt not. Therefore, it is not proper for thee to say to me,--Bestow thy sister. The custom to which I allude is our family custom. With us that is a virtue and worthy of observance. It is for this only, O slayer of foes, I cannot give thee any assurance in the matter of thy request.' On hearing this, Bhishma answered the king of Madra, saying, 'O king, this, no doubt,' is a virtue. The self-create himself hath said it. Thy ancestors were observant of custom. There is no fault to find with it. It is also well-known, O Salya, that this custom in respect of family dignity hath the approval of the wise and the good.' Saying this Bhishma of great energy, gave unto Salya much gold both coined and uncoined, and precious stones of various colours by thousands, and elephants and horses and cars, and much cloth and many ornaments, and gems and pearls and corals. And Salya accepting with a cheerful heart those precious gifts then gave away his sister decked in ornaments unto that bull of the Kuru race. Then the wise Bhishma, the son of the oceangoing Ganga, rejoiced at the issue of his mission, took Madri with him, and returned to the Kuru capital named after the elephant.