Why do ballast controllers come with 2 connections?

[JaySparks]

I noticed that arcadia offers either a single or a twin connection for their new t5 ballasts. http://www.ebay.co.uk/itm/121399068467?var=420337561923

 

[Markw84]

You plug one on each end of the fluorescent tube - so you need two connections for each tube.

 

[JaySparks]

You plug one on each end of the fluorescent tube - so you need two connections for each tube.

No what I meant was. Why do some ballast come with the ability to have 2 tubes.

 

[Markw84]

Just as there are double regular fixtures - sometimes you need two bulbs. With this setup you could even have the bulbs in separate enclosures, yet only need one ballast. A good ballast to get the most out of your bulb(s) is expensive.