Updated: Aug 2, 2019
A looper is a pedal device, which allows an artist to lay down, save, store and play back a beat- pattern or an instrument chord/lead sequence.
The real fun begins when you dub over each track that you create. You can keep dubbing tracks on top of tracks. It's like a mini recording studio. (Excellent for casual performance, when you gotta appease the listeners with something novel and fun.) Make no mistake though; to get the best out of a looper, you really need to have impeccable timing with your material. (The recording industry refers to it as punching in or out and good engineers are very precise with it) It's not a stretch to say that you probably should rehearse your collection of dubs.
However, there is also plenty of room for ad-lib, if you don't mind blowing up your material before an audience... (And after their 3rd drink, who wouldn't love that anyway?)
Pros & Cons
The main obstacle in using a looper, is that the song structure of your material becomes limited somewhat to 'circular' or recursive phrasing. For example, its more practical to find a 3-4 chord pattern that intuitively resolves after each pass.
Conventional songs process through a linear path of intros, verses, bridges and middles, and outros, each with their own resolve. . If you try this with a looper, you'd have to get it right for up to @3 minutes (The accepted conventional song duration) between each dub. The odds of making mistakes increases exponentially.
It is possible to support full-length songs with some of the more advanced loopers in the market., But, It would have to be able to store, sequence and playback independent clusters of tracks. The amount of memory and cluster sequence management could get pretty arduous if you tried this with a full set list of songs. Most hour-long sets usually contain about ten to twelve songs. (There are other tools out there, better suited to manage that).
With a looper, you can lock on to a tasty hook and embellish on it. For example , you could lay down the chords to a Jimi Hendrix lead solo section of a song (i.e. "Along the Watch Tower")... and then go to town on it with your version of a lead solo. You could fill copious amounts of time with amazing guitar trickery, as the looper cycles through and keeps repeating that song section.
If you have an advanced device, you could effectively create musical backing ... and feature your self as a very rich one-man band act. (Albeit, there are much better tools for that)
Using a looper is also an excellent way to practice and rehearse any instrumental 'virtuoso task' in your act, without the need or presence of supporting band-mates. In most cases you can use a looper to prepare for track bouncing and dubbing in the studio. (Ultimately, a cost saver.)
I just acquired a very basic looper pedal. Its making me get woke about my timing and meter skills. Nothing saves more money in a studio, than getting a track right, on the first or second pass.