In the section ‘Addresses’ of the settings you can determine whether addresses of points are looked up automatically.
If you enable the setting ‘Find automatically’ addresses of locations can be looked up if necessary. To find the address of a location you need an internet connection. The Topo GPS app is using the CLGeocoder service from Apple for this.
If you want a high level of privacy, you might want to disable this feature, in order to prevent network requests containing your location or the location of your points.