Some viruses display symptoms, and some cause damage to files in a system they have infected. But neither symptoms nor damage are essential in the definition of a virus. A non-damaging virus is still a virus, not a prank. A virus is surreptitiously self-installed. It may modify other software in the system without user awareness, and removal can be difficult and costly.
Many viruses cause intentional damage.
But many more cause damage that may not have been intended by the virus author. For instance, when a virus finds itself in a very different environment than that for which it was written, a non-destructive virus can suddenly become very destructive. Even if a virus causes no direct damage to your computer, your inexperience with viruses can mean that damage occurs during the removal process. Many organizations have shredded floppies, deleted files, and done low-level formats of hard disks in their efforts to remove viruses. Even when removal is done perfectly, with no damage to the infected system or files, it is not normally done when the machine is first infected, and the virus in that machine has had a few weeks to spread.