One tap from your home screen, and notifications the moment something needs you. Removed it by mistake? Install it again here: nothing is lost, everything is in your account.
No install button, and nothing in the menu? You may already have it: look for the icon on your home screen or in your app list. If you removed it, wait a few seconds on this page and the button above appears again; if it still does not, follow the steps for your phone.
Do I need the app for notifications? No. On Android and on a computer you can turn notifications on right in the browser. On iPhone the app is needed.
Your notifications and settings are kept in your account, so reinstalling loses nothing.