Add to Calendar will be automatically launched if you try to open a .ics file from your email or download one via your browser. This latter feature is especially useful as the .ics format is commonly used to provide a "download to your calendar" on event pages.
Add to Calendar will pre-populate a new appointment screen with the information from the .ics file, at which point all you need to do is tap the save button. If you wish you can also change the calendar to which the appointment is saved.
Add to Calendar fills one of Windows Phone 8's missing feature points and ensures you are more likely to have that vital reminder in your calendar. There are a number of other apps that offer similar functionality, notably iCalendar Import and ICS Helper, both of which are also free to download.
It's worth noting that all of these apps are import only, they will not "subscribe" you to an .ics file, which means if the meeting details change your calendar will not be automatically updated (in most instances you'll likely be emailed an updated .ics files), nor will the calendar appointment have information about attendees. Furthermore the apps will not reply to the contents of the .ics file, which means you may need to manually accept or decline an associated meeting invitation (e.g. calendar appoints sent out by Google Calendar arrives with an .ics file attached and separate links for accepting / declining the meeting).
Windows Phone Store description:
Open .ics files from your email or browser and copy them to your calendar.
Add to Calendar is a free download from the Windows Phone Store.