Spyware is all about collecting information. Eventually enough information becomes available to steal hardware (use other people's computers to run your scummy irc channel or bitcoin hive or piratebay, use other people's phone numbers, etc). Or to steal money. Or even to steal identity.
In practice, it's not about targeting individuals. Few of us are interesting or valuable enough to be worth the effort, our biggest protection is sheer boring anonymity. But if an automated keylogger or netlogger or whatever can get access to a password - then pass it upstream somewhere on the cloud - the information can be used to remotely access (login to) whatever that password locks out.
Voicemail and email are preferred targets because people often use them to interact with other things which can be worth stealing - like their banking and billing and "i forgot my password, please email me a new one" backdoor. Email accounts are also hijacked by (or sold to) spammers, they mail out to all visible email contacts - usually friends, family, and people you do business with - because they'll tend to trust "you" and click-install-run whatever (malware or spyware) attachments "you" send them.
People steal software product keys/licenses, online gaming accounts, pretty much anything they can, anything that costs money. As often as not, malware isn't about theft but simply vandalism, somebody gets kicks from wrecking everyone else's tech.
https://www.tripwire.com/state-of-security/security-data-protection/cyber-security/minidionis-voicem...
http://searchsecurity.techtarget.com/answer/How-does-the-new-voicemail-phishing-scam-work
Even if not malware/spyware, at least running scans can rule the possibility out so that the solution to this weird voicemail problem can be reached more quickly. It could just as easily be technical problems on the voicemail-end of things, not on the user-device-end.
