Bobby pins are to help hold the style in place. They've been used for ages now. They're small, come in different colors, and fairly noticeable. They're slipped in where, say, a curl needs to be held tightly to the head but hairspray can't do that job--or, at least, can't keep it in place long-term (hours).
If you're talking about clips that hold the hair back then it's so the make-up artist can do the make-up after the hair has been done, usually.
Tissue is used behind hairclips and such so that the hair won't end up with weird crimps or bumps from the amount of pressure the hair clip creates. This are usually only used when the make-up artist is doing the woman's hair so the hair isn't in the face, can be moved back, and not end up causing the hair to be oddly wavey when they're removed after the make-up is applied. It just helps to add a little cushioning so that the hairstyle isn't harmed. [ Peeps's advice column | Ask Peeps A Question ]
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