Lasso selections are useful to freeze a selection—usually multiple notes or dynamics—and adjust or copy them.
On Windows: To make a lasso selection of notes, hold the right-click button on the pen and draw a lasso around the notes. You can release the pen and the notes will remain blue. From here you can raise or lower the pitch of the notes by dragging them up or down.
On iPadOS: iPadOS users will see a lasso icon in the toolbar 
. Tap this to enter lasso select mode. When in this mode, draw with the Pencil around the elements you wish to select. Once you lift the Pencil from the screen, the selection is made and you can immediately start manipulating or copy/pasting that selection. You can also enable the option to use a double-tap gesture on the Pencil to trigger lasso-select from the score's settings panel.
If you've ever used a photo editing application, it can be useful to think of the lasso selection as a sort of "clone stamp" style tool. You can lasso select small groups of notes (for example, 4 semiquavers) and then quickly "stamp" them around the score.
Quickly repeat dynamics and text too!
Lasso selections work on dynamics, groups of dynamics (including hairpins), as well as text too! This can be a very convenient way to quickly copy dynamics to other instruments, or quickly apply text instructions to every staff in the score.
When a lassoed selection is made, the Reverse Stems 
icon appears in the command bar. Tap it to reverse the stem direction of the selected notes. (You can also reverse the stem direction by selecting an entire bar; more in Working with selected bars.)
You can quickly paste a lassoed selection to another stave or bar by tapping where you would like to paste them to.
Paste into Voice
Lasso selections are voice sensitive!
To paste a lassoed selection to another voice, first make the selection. Then tap the chevron 
at the top left corner of the screen to switch to the Toolbar. Choose which voice you would like to paste your music into, then tap in the bar.
To exit lasso mode, either double-tap (with your finger) in a blank area of the score, tap the Deselect All 
icon in the toolbar, or press the escape key on your keyboard (or the § key if you're on an iPad Magic Keyboard).