It encrypts your metadata (the most important data) and doesn't use it to manipulate you. It's a non-profit. And now you can use it without exposing your phone number to other users.
Again: Metadata. WhatsApp records a timestamp of every message you send/receive, and who the other party is. Signal only records two pieces of metadata: timestamp of when you signed up, timestamp of the last time you sent a message.
Whatsapp only e2e encrypts message contents. The only thing Signal knows about you at any given time is the time of account creation and the date of your account’s last connection to Signal servers. That's tied to your phone number. They don't know who you chat with, the contents of those messages, your phone contacts, anything.
I'd get a chuckle out of comparing that with the privacy of Whatsapp.
My 2¢, as someone who tried using WhatsApp once and ran away screaming:
WhatsApp requires you to give it access to all your contacts (your entire address book) in order to use it at all. This information is uploaded straight to Facebook’s servers where they’ll inevitably use it to place your WhatsApp account in a social graph so they know who you are based on your contacts. I found this to be unacceptable so I uninstalled it.
Even if all the other things sibling posters mentioned didn't exist, the simple fact that Whatsapp is owned by Meta and Signal is not... well, that'd be enough for me.
1. Facebook owns WhatsApp and uses it to collect data about people, such as who they communicate with, how and when. They also know about many of the websites you visit and what you do there. They know everything you do on Facebook, Facebook Messenger and Instagram. They buy mountains of data about us from other sources. By analysing all of that data they can probably do a reasonable job at guessing the content of your WhatsApp messages.
2. WhatsApp tries to get every user to accept the option to backup messages and photos to Google Drive, where they sit unencrypted and accessible by Google. Even if you reject that option yourself, your correspondents are likely to have enabled it (if only just to stop WhatsApp from nagging about it) and so your messages are available for Google to read. Example of why this can be bad: https://www.vice.com/en/article/zm8q43/paul-manafort-icloud-...
3. Google Photos asks WhatsApp users if they'd like it to back up their WhatsApp photos. Even if you reject that option, your correspondents may have enabled it and so your photos are stored online unencrypted and accessible by Google.
4. Why should we limit what Google and Facebook know about us? Google and Facebook influence our behaviour for the benefit of their paying customers. Their computer systems are too powerful for our minds. They work against us, not for us. Companies like Facebook will come to be seen like tobacco companies, except that the harm is as from mind altering drugs. There is a documentary on Netflix called The Social Dilemma which explains this well. The polarisation of societies and the spread of conspiracy theories are some of the effects. The only defence is to disengage.
Edit: Ok, ok, I was wrong, signal does have advantages over whatsapp.