Going into flowering

Should i always wait until i see preflowers before i go into 12/12?

No. You can flip 12/12 whenever you want.

What are the benefits of vegging long? I assume there are some benefits?

the plants grows bigger before it begins maturing, as a result it grows more branches and therefore more bud site resulting in more bud. keep in mind the bastard can double or treble in size so dont veg too long if your grow area is of limited size. trust me most of us have made that mistake and ended up with damn trees poking out closets and wondering if we're gonna have to cut holes in the ceiling.

after a few grows you'll see how that when you initiate the 12/12 the plant will slow its upward and outward 'vegetative' growth -
if you are outside growing then you try to plant as early as you can so that they have 2 - 3 months before nature turns on 12/12 - it can mean the difference between a 2 foot and a 12 foot plant outdoors..

How long you vegetate your plants largely depends on the size of your grow area and the intensity of your lighting. If you're using high-wattage HID lighting and have ample growing space, you can grow monsters with tremendous yields. You could reasonably vegetate for 8-10 weeks and induce flowering with 4 foot plants. A 1000W HPS could give you colas the size of your forearm in this scenario. However, if you're limited for space and/or using florescent lighting or led lamps, you'll need to keep things contained. These lights simply don't have the intensity and canopy penetration to flower such large plants. Induce plants to flower, when they're between 12-18" in height.