The answer is “it depends on your use case”
If you need to connect your on-premises VoIP solution to Teams so that your users can call between systems without breaking out over the PSTN, then yes you need direct routing for those internal voice calls.
If you are moving to Teams and calling plans as a business and do not need any interop between VoIP systems and Teams, then probably the answer is no here.
Sometimes however, it may be that you have multiple offices in different countries. Users in the UK may spend a lot of the time calling USA numbers. Providing international calling plans, or PAYG minutes could be expensive at scale. However, if you have a PSTN breakout in the USA office for instance, you could use Direct Routing to route USA bound numbers across your WAN using Direct Routing and pay a local rate charge instead of a calling plan that covers multiple destinations and a bunch of minutes you may not need.