It is the Sunnah of Prophet Muhammad SAW to offer supererogatory prayers until Imam arrives. So, going early allows you to offer nawafil as many as you like.
But some of us don't get to the Mosque until the Imam is about to round up the Kbutbah. At this point, can one still offer supererogatory prayers?
Here is the explanation.
As stated in Fiqh Sunnah, once the Imam comes in (and greets the congregants) one should no longer offer any Nawafil except Tahiyaytul Mosjid (the prayer of greeting the mosque) which may be performed quickly during the khutbah unless one comes at the end of the khutbah and would not have the time [i.e.,before the actual salah begins] to perform tahayyatul masjid.
It is reported that Ibn 'Umar used to perform along prayer before al-Jumu'ah and then two rak'at after it, and he said that the Prophet used to do so. This is related by Abu Dawud.
This hadith sums up the significance of observing supererogatory prayers before Khutbah begins.
Jabir reports that a man came to the mosque on Jumu'ah while the Prophet was delivering the khutbah. The Prophet inquired of him: "Did you offer the salah?"The man replied: "No!" He told him: "Pray two rak'at." This is related by the group. In one narration it states: "If one of you comes to the mosque on the day of Jumu'ah and the imam is delivering the khutbah, he should pray two rak'at and make them quick." This is related by Ahmad, Muslim, and Abu Dawud. In another narration, it is stated: "If one of you comes to the mosque on the day of Jumu'ah and the imam has already arrived, he should offer two rak'at." This is related by al-Bukhari and Muslim.
The best for us is to leave home early and go straight to mosque so as to have enough time to engage in some acts of Ibaadah. May Allah accept our supplications.