Firewalls can do much more in controlling traffic versus a typical router. A basic ASA firewall will be at the very least be a Statefull firewall, which looks for an active session before allow traffic back in the firewall. This means it is unnecessary to create reflexive rules which may be too broad for traffic coming back to the firewall. If you are using an ASA as a NSFW (Either FTD or ASA w/Sourcefire) now you can create rules that reference Layer 7 information instead of Layer 4 information. In short why use a Firewall for ACLs versus a Router? Because a firewall is much more flexible due to it's statefull firewall nature.
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