Besides client side (javascript) encryption, Braintree supports Transparent Redirect (https://www.braintreepayments.com/developers/api-overview) which lets you post data directly to Braintree.

The nice part of sending the data to your servers with client side encryption is that you can do validation before sending to the payment gateway. For example, if you want to ensure everyone enters a cardholder name, you can validate the non-encrypted fields before eating the cost of calling a payment gateway.

You can do some of these validations in javascript, but javascript is error-prone (firebug) and not as flexible (have access to a lot more data server side).