In the context of defining what comprises the special Ne’ila prayer on Yom Kippur, the Gemara on our daf brings a baraita that discusses people who go to the mikvah to immerse themselves on Yom Kippur. (Remember that washing is one of the five inuyim – activities forbidden on Yom Kippur if they are done for pleasure – that are enumerated at the beginning of this perek, see 73b.) According to the baraita, anyone who is obligated to immerse in the mikvah can do so on Yom Kippur, based on the principle that tevila bizmana mitzva – there is an obligation to purify oneself in the mikvah at the time when one is able to do so.
The baraita continues: One who had a seminal emission during Yom Kippur before the afternoon prayer may immerse at any point in the day until the afternoon prayer, in order to be able to recite it.
The obligation to immerse in the mikvah in this case is based on the assumption that a person cannot participate in prayer or Torah study if he does not purify himself, making such purification essential for participation in the Yom Kippur synagogue service.
This assumption stems from one of takkanot Ezra – one of the rules that Ezra ha-Sofer established during the early part of the Second Temple period – whose intent was to limit the amount of time that a committed Jew would spent engaged in sexual relations (even with his own wife). Already during the time of the later tanna’im and early amora’im (after the destruction of the Second Temple) this takkana was no longer kept for a variety of reasons. Still there are many who continue to keep this tradition even today. The Me’iri claims that, in his time, there were individuals who continued practicing tevilat (the immersion of) Ezra and would even go the mikvah on Yom Kippur itself, if necessary.