I thought that you always pinned your fabric pieces together before sewing. Then, on the Great British Sewing Bee, I saw several contestants using these little plastic clips and ordered some straight away from Amazon.
WOW these are so useful and help get pieces aligned much faster that using pins.
Pins still are essential but the clips hold the pieces together, help with repositioning and in some cases you don’t need the pins at all.
Sometimes I use the clips first and then pins secondly. It changes on a case by case basis.
Both are useful. As I am developing my sewing skills, I mix and match according to what I’m trying to do. Inserting sleeves is a great challenge. You’re trying to add a curve within a curve. This is where the clips come in really handy just to get everything spaced out. Then, I pin it all together and it makes the whole job so much easier! 
The clips shown here are from amazon.co.uk and the product link is this.
Please comment on these clips, I think they’re great!
Stephen Sews 