Collecting rubber isn’t about adding weight. The teams want to mask the exact tyre wear amount/patterns, which they do by covering the surface with scraps. <ed

Collecting rubber isn’t about adding weight. The teams want to mask the exact tyre wear amount/patterns, which they do by covering the surface with scraps. <edit> Actually this seems less clear cut than I thought. I'd heard from David Coulthard's commentary that the reason for picking up was to cover the wear from other teams' eyes. A quick search suggests many people believe it's to do with weight. The weight explanation seems odd, since if the officials think the tyres are masking the true weight of the car, they can demand a different set are used for the weigh-in, but I can't find an authorative source either way. So it may be to do with weight, or it may be to do with hiding tyre wear. </edit>