Heiresses were bought and sold
In medieval times, the sovereign and his tenants-in-chief had problems in raising revenue. They may have had the rent from their own estates but these were seldom enough to fund their many ambitions and duties. But the core principle of feudalism was that land was held of the king by the tenants-in-chief and likewise by lords of manors of a tenant-in-chief. The feudal superior had the right to withhold the land, though this tended to be highly unpopular if done in an arbitrary manner. But what was accepted was the charging of a fee when the land passed from one holder the next.
So heirs to land paid a fee, usually called a fine (=final payment), to their feudal superior when they inherited the land. Today we would call this death duties. This was fairly easily handled when the heir was a male adult, he paid the fine and carried on where his father had. The feudal lord got his fee quickly
But for minors and heiresses the practice became that the feudal lord took them into their household and collected the fee when they became adult. This delayed the receipt of revenue. So they asked others to bid for the guardianship and surrendered the right to determine the marriage to the new guardian. This brought the revenue in more quickly.
These guardians bought and sold heirs and heiresses. While their charges were young, they received the revenues of their estates. If they bought a male heir they tended to marry them to their daughter, thus providing for their daughter as the mistress of an appropriate establishment. If they bought a female heir, an heiress, they could determine who she was to marry and again might well marry them to their son, thereby enlarging their son's estate.
Obviously there was some friction here and it was not unknown for the proposed marriages not to occur; the prospective spouses did have the liberty to refuse to marry, but it would be a brave youngster, of 12 perhaps, who would exercise this supposed freedom.