Key Sequences (KS) are truly a gem in the very rich treasure trove of BTT. I’m not aware of any other app that does this so well. If I had to choose just one section in BTT, it would be KS.
I’m not at my computer right now, so just quickly: take a look at this. These are things I do with every modifier. For example, this is how I launch my most-used apps.
The Power of BTT - Configuration Help & General Discussion - BetterTouchTool Community
If I list things randomly: left and right cmd are the modifiers you can reach most easily and blindly with your left and right thumbs. So, a useful KS would be: hold left cmd and tap right cmd to trigger something (or vice versa). I use this to navigate tabs left/right in the browser. Another simple method is holding both cmd modifiers and triggering something with j, k or f, g keys — but those are shortcuts (not KS). I mention this because many people don’t know that a shortcut can also be set up with two identical modifiers, like cmd + cmd + letter.
I find KS that work with a swipe especially practical. For example: left shift and control. You can simply swipe your index finger from top to bottom. Depending on the keyboard, this also works horizontally.
As far as I know, KS are the only way to assign a triple function to a key combination. For example: left cmd + space, left cmd + space + space, and left cmd + space held down. Of course, even more variants are possible, but at some point it becomes absurd. For others, not for me. 
And of course, KS are ideal for text expansion. I do it like this: ‘btt space space’ expands to ‘BetterTouchTool’. Others use complicated prefixes with rare characters. Double space at the end seems enough to me, as I never use two spaces in a row in my texts.
In connection with the BTT “Keyboard Block” function, KS are the only way to trigger something with whole words (or parts of words). Note, this is not text expansion. What you type is only passed to BTT (not the front app, which is very useful!) and BTT can then send it to any app. So you can type something from anywhere that won’t be typed, but will trigger whatever you want.
And finally (just between us 
): KS are the last resort to work around bugs with shortcuts. These are bugs that I don’t even report, since they only show up if you do strange things like I do.