Tim - First off, you are definitely not stupid, and neither is your question. The very question you have posed was one of my very first
major discoveries as a guitar player. Let me shine some light on it for you.
For starters, the Valve Jr. is not "just" a 5W amp - it's a 5W Class "A" all-tube combo. Some of the greatest music ever created first eminated from combo amps very similar in design to the Valve Jr. One that readily comes to mind is Led Zeppelin I, with Pagey playing a vintage-era Telecaster through a little Supro amp. As tonally earth-shattering as that album was, it is a mere speck of sand on the beach of contributions that small tube amps have made to music as a whole. Players in all generes of music have employed their much sought-after tones - blues, jazz, rock, country, alt. rock, pop, etc. The list goes on forever. Since these amps have been around since the beginnings of electrified amplification, they are truly a step back into the past as far as sound and tone are concerned. Yessir, the vintage "vibe" of these amps is totally unmistakable. I am not an electronics whiz, so having me explain the nuances of Class "A" tube amplification vs. the Vox ADVT series amps would probably not do justice to either. They are two totally different animals - one employs technology well over a half-century old, and the other is on the cutting edge of modern hybrid / digitally-amplified circuitry.
So how does this cool tone happen? Well, it goes something like this: a tube amp with a lower wattage allows you to "drive" the tubes to their natural saturation point, and therefore, obtain that sweet, organic tube distortion so many guitarists dig - at a volume level that won't peel the paint off the walls. In a higher-wattage amp, let's say a 100W Marshall, you have to crank the volume to get those same tube distortion characteristics, which for some, isn't a bad thing. A small-watt tube just lets the same thing happen at sub-Hendrix-at-Woodstock volume levels. Granted, this is a layman's explanation of the effect. There are many more knoweledgable, gear-oriented Fret.Netters than myself, and hopefully, there is more follow-up to your post than just myself.
I still remember the day I posed your same question to one of the guys down at my local shop - probably 1985 or thereabouts. Being more or less enamored with metal music at the time, I scoffed at the idea of a small combo having anything to offer the modern hard rock player. He told me to stop down after school the next day. So my friend and I did. He brought his 50's era Tweed Fender Champ (a small-watt tube combo amp), plugged it, let it warm up a few minutes, cranked the volume, and said, "boys, listen to this." To demonstrate the awesome effect of this amp, he pulled a nice dual HB B.C. Rich Mockingbird (a beautiful deep wine-red metallic guitar w/ abalone inlays - a guitar which my friend eventually bought) off the wall, and proceeded to blow our minds with some of the most savagely distored heavy metal licks our young, solid-state brains could comprehend. It was pretty damn loud, too! My friend and I, of course, were floored, and from that point on, we've both had a whole new appreciation for vintage-era tube gear. By the way, that guy still owns that amp, and it sounds even better today. I demo'ed a G & L Tele exactly like the one I have now through that same rig, and I must say, it had THE coolest Stones-y tone I've ever heard. He also enjoys relaying that story to friends and customers at every possibly opportunity, after of which he will demand that we buy him a beer for, as he would say, "showing us the light"! Indeed.
There numerous sites on the 'web that compare tube tone to solid-state, modeled, and hybrid amp tones. The first thing I would suggest is that you do a little research, and familiarize yourself with the inner workings of each type of amp. And since tone is a relatively subjective issue, I would encourage you to head down to your local guitar shop, and play a few low-watt tube combos yourself. Your take on it will likely be far different from mine.
I always find it more rewarding to learn things like this for yourself, rather than have someone else tell you about it. The tube "thing" is truly one of the most "ear-opening" experiences I've had in my (23) years of guitar playing. It is also my hope that this "discovery" opens some new doors for you, and your own playing. It certainly did for me.
