The message MediaWiki:sig-text is used by both the old toolbar and WikiEditor to determine how a signature should be formatted. It defaults to "--$1", where $1 is replaced with "~~~~" The two dashes serve as delimiter, separating the signature from the comment.
VisualEditor should be consistent with those two other toolbars and use this message when the user inserts a signature via the tool from the toolbar, both in visual and wikitext mode.
This has already been reported by @Piramidion as part of T153255: Promote the signature button to the toolbar (or somewhere else more prominent) in VE/NWE, but let's have a separate task for this.