Daf Yomi - Yoma 87 - Maariv is Only voluntary if .....

Rav holds that the tefillah of neilah, the final tefillah prayed on Yom Kippur can be recited at night. Therefore, he rules that if one davened neilah after dark, he will be exempt from davaning maariv that night. His reasoning is neilah is considered an extra tefillah and since one davened a tefillah at night, that will be valid for maariv as well, which is a tefillah that is normally recited at night.

The Gemora asks on Rav that he rules in Berochos that the tefillah of maariv is only voluntary and not required, so why would our Gemora say that neilah exempts him from maariv - he is not obligated anyway? The Gemora answers that his ruling here would be relevant to the one's that hold maariv is mandated.

Tosfos quotes the opinion of the R"i that maariv is only voluntary if he has other pressing needs at the time, such as performing a mitzva that cannot be done at another time, but if there is no other urgent necessities at this time, he would be obligated to daven maariv.

Tosfos asks that according to the R"I, what is our Gemora's question? While it is true that Rav holds maariv is only voluntary, that is only in circumstances where he has other obligations but normally one would be duty-bound to daven maariv. He answers that on Yom Kippur night, there is automatically pressing needs such as the preparing of the meal for after the fast. The Midrash states that the meal after Yom Kippur is tantamount to a Yom tov meal and there would be a mitzva to involve oneself in the preparation. This would cause that the davaning of maariv is only voluntary.