Pledging charity during Yizkor, and then giving it after Yom Tov, helps bring atonement for the soul and is the main purpose of Yizkor.
See Rama Y.D. 249:7
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Pledging charity: [1]
One is to pledge charity during Yizkor on behalf of the soul of the deceased. Doing so helps bring atonement for the soul, if the deceased person would have given the charity if he were to be alive.[2] [This is one of the main purposes of the Yizkor. One is to explicitly state in his pledge that he is doing so Bli Neder, otherwise it has the status of a Neder.[3] It is proper for the pledge to be paid immediately after Yom Tov.[4] Some are accustomed to already distribute the charity before Yom Tov, and then say in the Nussach “As I have given.”[5]]
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[1] Rama Y.D. 249:7 “The custom of pledging charity during Yizkor is an old custom and it helps the soul of the deceased”; So rule regarding Yom Kippur: Admur 621:14; Michaber 621:6; Rokeiach 217; Mordechai Remez 727; So writes regarding the Yizkor said on Shabbos: Shibulei Haleket 81; Beis Yosef 284; Bach 282; Nitei Gavriel Aveilus 79:127
[2] See Admur ibid; M”A 621:6; Taz 621:4; Rokeiach ibid
[3] Michaber Y.D. 203:4; Kitzur SHU”A 67:3; Nitei Gavriel 79:28
[4] See Alef Hamagen 581:102; Nitei Gavriel 79:30
[5] Gesher Hachaim 31:2-3; Nitei Gavriel 79:29