Actually, anything that outputs a constant beat can be used as a clock if you have something which can extract that audio and convert it into clock pulses. All you'd need is a very tight bandpass filter (as in 1/10th octave or tighter) and the conversion tool of choice (I use a Truetone Time Bandit myself), and you could lock your DAW (or most anything else) to some ancient 1960s drum machine or even beats on prerecorded audio. Been doing variations on this for years, and it works pretty well.
As for an additional clock or modules to modify clocking, yeah, you DO want those. Let's say, for example, that you want an event to hit every other measure on beat 3. Your track is in 4/4, you can easily get a quarter-note pulse, your sequencer has 16 steps, and your loops are 4 measures. You COULD tie up a sequencer channel to do this, true...but the more efficient solution would be to use something like an EMW Pulse Counter. Send the quarter-note pulses in, send a pulse from steps 1 and 9 to the Reset function, and take your specific trigger pulse out of step 7 for...well, most anything. Or send your clock gates into one side of an AND gate, and then the output of a comparator to the other, and set the comparator so that as long as the note CV coming out of the sequencer is over a certain pitch, the AND gate will pass your pulses on to another sequencer ONLY when the "high state" is present both from the comparator AND clock. Neat way to add a tessitura line on your sequence line AND have the machine do it so you can twiddle with something else.
...and so on....