This can be understood from the following facts:
As explained above that the reward is unlimited.
It is better to wait for a few moments and join Jamaat Salaat than to pray alone on actual time. (Awwal-e-wakt)
If one completes his Furada Wajib prayer and finds that Jamaat Salaat has started, he can join Jamaat Salaat and pray that same Salaat again, thus he get more Thawaab (reward).
If a father & mother order their son to pray Jamaat Salaat, and by disobeying the order, he is afraid of upsetting them, then it is Wajib upon the son to pray with Jamaat.
Those who do not know proper pronunciation or recitation should pray with Jamaat to learn proper recitation.
According to certain Ulema one cannot pray Furada while Salaat-e-Jamaat is in progress.
If one does not consider a particular Imam as "Aadil" and he is already inside the mosque, then he should either pray Wajib Furada between Adhan and Ikamah or wait until Jamaat Salaat is finished, as a mark of respect.
Increase of Rukn either intentionally or unintentionally make prayer invalid, but in congregational prayer such increase is allowed due to significance of Jamaat Salaat (Details later on).
Performing salah in congregation is Sunnah Mu'akkadah, or a confirmed Sunnah. Many hadiths discuss the virtue and importance of performing salah in congregation. Here are some of them:
Ibn `Umar quoted the Prophet (peace and blessings be upon him) as saying: "The reward of salah performed congregation outweighs that of salah performed individually by twenty-seven degrees." (Al-Bukhari and Muslim)
Abu Hurayrah quoted the Prophet (peace and blessings be upon him) as saying: "The prayer of a man in congregation is twenty-five times more superior (in reward) to his prayer in his house or the market – and this is because he makes the wudu' (ablution) and perfects it, and goes to the mosque with the sole intention of performing salah. He does not take a step without being exalted in degree and having one of his sins obliterated. When he prays, as long as his wudu' is still valid, the angels continue to pray [for him]: 'O Allah, bless him. O Allah, bestow Mercy on him.' He is considered as if he is performing salah as long as he is waiting for the salah." (Al-Bukhari and Muslim)