You do not NEED to get an expensive thermometer like a Fireboard (but you can if it fits your budget). What you NEED is thermometers than read the temperature accurately. Nearly every member of this forum will tell you that the Traeger thermometers are not accurate.
Your actual pit temperature will read either higher or lower than the controller temperature. I place a digital thermometer probe just above the grate upon which my meat is cooking got get an accurate temperature. Every grill will be different, but on my grill, I set the controller at 230F to achieve a grate temp of 225F. I need to set the controller to 325F to get a grate temp of 300F. To get to 400F, I need to set the controller to 450F, a full 50 degrees higher than the cook temperature. Without the external thermometer, I would never know cook temperature was off.
You can compensate for inaccurate cook temperatures by increasing or decreasing cook time. On the other hand, inaccurate internal temperature readings can either leave your protein overcooked, tough and dry, or perhaps even leave it undercooked and unsafe to eat. I used the Traeger internal meat probe for my first cook, but then decided to purchase 3rd party thermometers. Now, my Traeger probe remains in its storage spot in the hopper, never to be plugged in.
For cooking briskets, roasts, etc. an internal meat probe is ideal as you can monitor the progress of the cook.
For fish, chicken breasts, and steaks, an accurate instant read thermometer is ideal. Be aware that there is no concensus on the meaning of "instant". Some such thermometers read in 1-3 seconds, which is wonderful. Others can take 10 seconds or longer, which can be frustrating when the the lid of the grill is open allowing heat to escape. Be sure to get one that will provide results in less than 3 seconds, or certainly no more than 5 seconds.