Google Drive is Google's cloud storage. It stores your documents, spreadsheets, and files online, lets you access them from any device, and allows you to share them with whoever you want.
The Google Drive CRM integration lets you attach Drive files to your contacts in OnePageCRM, so the proposal, the contract, and the spreadsheet are on the record they belong to.
Because the conversation and the paperwork belong together. The quote you sent, the signed agreement, and the spec sheet all relate to one customer, and this puts them on that customer's record.
No. Files stay in Google Drive, and the CRM links to them. That saves your CRM storage, and since everyone opens the same file rather than copies, nobody works from an outdated version.
Sign in with your Google account when attaching your first file. There is no separate app to install. The setup guide covers it alongside the other Google integrations.