Firstly, with the right equipment, your wireless data transmissions can be intercepted in the same manner that a radio receiver can be tuned into radio station broadcasts. If your wireless network doesn’t use encryption, an attacker could easily read that intercepted wireless data. If the “data” consists of usernames and passwords, credit card and bank information, sensitive documents and personal data, the attacker has all they need to commit ID fraud.
Secondly, if no authentication is used, an attacker could join your own wireless network and commit bandwidth theft - use your Internet access to launch attacks on other networks or to download illegal content. In that case, the ISP and law enforcement would come knocking on your door rather than the attacker’s.
Thirdly, if the computers using your wireless network are insecure (poor passwords, for example) an attacker could use the wireless network to access that computer directly - they could then obtain whatever documents and information that they wish, use it as a store for planting illicit material, or plant a Trojan for example. The problem is that “out-of-the-box” wireless equipment is not generally configured in a way as to prevent such attacks.
Whilst organisations access the administrative interface of wireless devices and configure them securely before they are deployed, the majority of home users simply plug and play such equipment with no awareness of the risks! Such home users are then easy prey for war drivers and other miscreants. So get out that user manual and secure your wireless devices before it is too late!