Filtering of BGP prefixes is important when peering with EBGP neighbors, lest you become a transit AS.
Consider the following example:
If filtering is not applied on GW1 in the outbound direction, then ISP1 may receive routing information about the network update originally sent by ISP2, effectively making AS 65000 a transit AS.
Filtering can be applied in the inbound or outbound direction. Route maps, prefix-lists, and AS Path filters can be leveraged for filtering purposes. Usually, route maps and prefix-lists are used for this purpose.
Stuck-in-Active: Journal of an IT-Network Administrator 