Well they would say "think that's me? Prove it" which they probably can't from CCTV alone, but if a name is repeatedly put forward as an identification or a witness in the street gives a better description and says "I saw that guy, he looked like this..." then the case against him starts to tip in the favour of police.
They have identified a suspect but proving it is difficult, and if people ie. Family members say "he was home the whole time" then that becomes a frustration for police. They need to build a case, and this CCTV would support other evidence.