The overwhelmingly most important advantage of learning in Kollel is that you are learning. Simple as that. Torah learning is the highest, greatest, most glorious form of Avodas Hashem, which will get for you the greatest share in Olam Habah possible, much much more and much much greater than any other thing you can do for Hashem, including any other type of Mitzvos. Learning Torah brings by far the greatest nachahs to Hashem, the greatest benefits to Klal Yisroel, and the greatest benefits to those who do the learning, than anything else you can possibly do. Hands down, no contest. That’s why we learn.
Talmud Torah Kneged Kulam. But it’s not a question of Halachah, not l’chatchilah not b’dieved. It’s simply this: If I showed you a pile of coins and gave you 1 hour to collect as much as you can, you would spend as much time gathering the gold as you could. Torah is the greatest Mitzvah – one word of Torah, Chazal say, is more holy than an entire lifetime of doing Mitzvos. And we have one lifetime to gather our gold. Someone who appreciates that doesn’t care if he is halachicly obligated of not. Everyone agrees that learning Torah is gold. Even if you are not halachicly obligated to run after it.
There are tons more good reasons — like the effect it has on your home, on your Mitzvah performance, on your personality, on your children — all of which are excellent reasons. But the main reason is, that our goal in this world is to go higher and higher. And there is no better way to go higher than to learn. Nothing even close.