During the buying phase, communicate with your team and make a plan. The team-based design of CS:GO means that your actions heavily impact your team. You shouldn’t only do what you want to do, but you should do what the team needs you to do.
If everyone on the team executes their part of an overall strategy, your team is stronger for it. The downtime before a round starts isn’t only for buying your loadout, it’s also time for establishing a strategy for the round. Particularly if the plan is to rush an area, the pre-round time is vital to get the team coordinated.