Use of sales_quote_address_collect_totals_before events Magento 2.

You can use sales_quote_address_collect_totals_before event for setting your custom totals related data before Magento collect final totals for a quoted address.

Use of address collects totals before event inside observer, You can fetch specific quote data, shipping_assignment, and total related data for a cart.

The event is defined under the Magento_Quote/Model/Quote/TotalsCollector.php file of the Quote module.

You can be interested to retrieve  A Complete List of all events in Magento 2.

Using Event, You can set your custom data before the quoted address set final data.  Define events in events.xml file,

<?xml version="1.0"?>

<config xmlns:xsi="" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="sales_quote_address_collect_totals_before">
        <observer name="your_event_observer_names_defination" instance="Jesadiya\TotalsBefore\Observer\TotalsBeforeEvent" />

Create an Observer file to handle your custom data,

namespace Jesadiya\TotalsBefore\Observer;

use Magento\Quote\Model\Quote;
use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;
use Magento\Quote\Api\Data\ShippingAssignmentInterface;

 * Class TotalsBeforeEvent
class TotalsBeforeEvent implements ObserverInterface

     * @param Observer $observer
     * @return void
    public function execute(Observer $observer)
        /** Fetch Address related data */
        $shippingAssignment = $observer->getEvent()->getShippingAssignment();
        $address = $shippingAssignment->getShipping()->getAddress();

        // fetch quote data
        /** @var Quote $quote */
        $quote = $observer->getEvent()->getQuote();

        // fetch totals data
        $total = $observer->getEvent()->getTotal();

The event is used for handling customer address data before the final address and total are set for a quote.

Using the event you can customization of your quote address related stuff in Magento 2.