Since Guam has more lax immigration rules than Honolulu (see Asian visa waivers above), the Guam-Honolulu flight has a semi-secured pre-board waiting area (your BP gets processed before entering this area) and a pre-board immigration semi-check. USCBP Guam makes sure you're eligible into the States and stamps your BP stub. Because of this check, you must arrive in the waiting area 45 minutes before scheduled departure or they'll unload your bags.

I say "semi-check" because at Honolulu you go through the other half. If you have the Guam CBP admission stamp, you go right through. I hadn't traveled this route as a non-US citizen, but I can imagine tourists and immigrants get the main-course processing at Honolulu.

Since arrival at Honolulu is on the evening BEFORE the morning one leaves Guam, this route has been utilized by "desperate" immigrants, e.g. a Filipino immigrant nearing his/her 21st birthday reuniting with family on Guam. On the eve of his 21st birthday, he boards a flight from Manila, arriving on Guam on his birthday (thus ineligible to immigrate into the US). However, he can then continue to fly to Honolulu, and unless there's a 6-hour delay, he's still legally 20 when he arrives in Honolulu where he gets processed for a Green Card.