Sometimes smaller is better
When you first run Secrets the main window will open with an item list on the left and a detail pane one the right. This layout is very common on the Mac, Apple's Mail and Contacts applications use it also. You select an item on the left to view its details on the right. For the majority of users it's a well-known concept.
This layout works very well when you need to view or edit your item's details. However, when designing Secrets, it was clear that most of the time the interaction with the application would be a quick and simple information retrieval, such as getting a password, a credit card number or filling a login in the web browser. In this scenario, a smaller window focused on searching would be a better fit.
That's why you can hide the detail pane since the very first version of Secrets. You can try this yourself by selecting View and then Hide Detail Pane in Secrets' menubar, or by using the keyboard shortcut ⇧⌘D.
Expanded vs collapsed mode in Secrets for Mac
You may be wondering how do you retrieve a password or credit card number if the detail is closed… Well, you just select the item you want and press ⌘C to copy the most relevant information for that item type to the clipboard. For Logins this will be the password, for Credit Cards the card number and for Bank Accounts the account number. Also, if you use the alternate ⌥⌘C keyboard shortcut you'll copy the username for Logins and the PIN for credit cards.
This collapsed mode also works great for when your filling logins in Safari. And if you have a small screen on your Mac you can make use of the Split View feature introduced in El Capitain to have Secrets and Safari open side by side in fullscreen.
If need to access some other information or edit an item you can open the detail pane again, obviously, or simply double click the item to show the item's details without expanding the detail pane.