Berachot 9b revisits the topic of סמיכת גאולה לתפילה, connecting the blessing of גאל ישראל which speaks of our redemption from the bondage of Egypt with the שמונה עשרה which had already been discussed on Berachot 4b. In Berachot 4b it said that anyone who connects גאולה and תפילה is inherits World to Come. Our Gemara goes one step further saying that one who connects גאולה and תפילה will have to harm befall him all day. Tosfot comments based on the context of the Gemara that our Gemara must not be referring to a regular person but one who prays ותיקין, timing Shema and Geal Yisrael immediately preceding the sunrise so they can start their Tefilla, שמונה עשרה, exactly at sunrise.
Rav Ezra Bick in a wonderful shiur on שמונה עשרה for the Gush Virtual Beit Midrash quotes the two approaches from Rabbenu Yonah on the benefits of סמיכת גאולה לתפילה. In summary, in the first approach Rabbenu Yonah says that by connecting the two, we are first acknowledging the Exodus from Egypt which makes us slaves to G-d and then immediately expressing our service to G-d through שמונה עשרה. Why שמונה עשרה? Because the שמונה עשרה expresses the fact that we are totally dependent on G-d for all of our needs. The second similar approach of Rabbenu Yonah is that after acknowledging the Exodus which was the basis for the total faith of the Children of Israel in G-d, we ourselves show our total בטחון in Hashem to fulfill all of our needs by reciting שמונה עשרה.
What is the extra added benefit from our Gemara of סמיכת גאולה לתפילה for ותיקין? I believe the answer can be found by looking at the verse that is quoted as a proof-text: יִירָאוּךָ עִם-שָׁמֶשׁ; וְלִפְנֵי יָרֵחַ, דּוֹר דּוֹרִים, They fear you (Hashem) with the Sun, and before the Moon for all generations. (Tehillim 72:5). One could explain that the ותיקין expressed their total servitude and בטחון in Hashem at the very first moment that we were redeemed and became G-d's slaves, which was the beginning of the day of the Exodus. It is this total devotion of the ותיקין which warrants its own special extra reward.
It is interesting to note that Rashi actually understands a similar concept by Mincha based on the second part of this verse. He says אף תפלת המנחה מצותה עם דמדומי חמה, even by Mincha there is a Mitzvah to recite it with the twilight. This is the source for the minhag that some Chasidim have to daven mincha very late even after sunset. According to this understanding, ותיקין express their total devotion to Hashem by serving and trusting G-d for the entire day, from the first moment of the day, exactly at sunrise, until the last moment of the day, which is more difficult to determine, which corresponds with twilight.
This could also explain why we only have a concept of סמיכת גאולה לתפילה for ותיקין in the morning and, according to Rashi, later during the daytime at Mincha but no similar concept at Maariv. We can only express his total servitude and trust in G-d at the moment that we completely left the yoke of our Egyptian masters and became totally devoted to Hashem. This only occurred in the morning after the מכת בכורות which occurred the night before. Only with the light of day did the Benay Yisrael begin to leave the bondage Egypt and enter their new relationship of exclusive service and trust in Hashem.