Inspired by this weekly challenge.
Why did Boaz choose to make yibum to get the field of Machlon and Kilion. Couldn't he just buy the field from Ruth?
It would appear that nothing here was in the bounds of absolute strict obligation. (The Torah only strictly obligates yibum for the brother of the deceased; Boaz and Ploni were more distant relatives.) Nonetheless, it was felt that to honor Elimelech's memory, it was appropriate that one person both buy his ancestral fields, so they'd be back in the family, and marry the widow. I guess if it's an honor, you don't go in halfsies.
According to the Malbim's commentary, Boaz chose to do Yibum in order to spiritually reincarnate Rus's deceased husband, Machalon, which is the Zohar's approach to what happens in Yibum - the soul of the deceased is reincarnated into the son born of the Yibum union. (See Malbim to 4:14, for example.)
Along the same lines, he may have chosen to acquire the field through Yibum instead of acquiring it separately in order that it be the nachala (inheritence) of Machalon, and not his own unrelated property, as the Malbim explains at the end of verse 4:5 that the field acquired through Yibum will be associated with the deceased.
The implication of the Malbim is, however, that Tov had no option to buy the field, and it was not merely a choice to acquire it through Yibum instead of purchasing it, but rather this was the only option of how to get the field belonging to Rus, as is clear in his commentary to the first part of verse 4:5. This could possibly be because (my own suggestion) Rus was unwilling to sell the field in order that it be a "bargaining chip" to have Yibum, especially once she knew that Boaz was willing to do Yibum once Tov stepped out of the way (verse 3:13).