Connecting your iphone or ipod on new computer or fresh installation of itunes don’t let you sync your music or applications.
It asks for erase and sync because iphone or ipod can be synchronized with only one itunes at a time.
When I did erase and sync first time, I googled it but according to many people-: contacts, notes and applications will be erased
And someone told me that my cydia application will also be get erased means i would have to jailbrake my iphone again.
Doing erase and sync will never erase your whole iphone.
It will just erase sync files with the previous iTunes library and will sync your iphone with new itunes library.
So don’t worry about applications to get erase or to jailbrake iphone again.