Most major brands use multiple dsps, as performance is easy at low spend volumes and gets more difficult as you spend more. Dsps have access to unique inventory

Most major brands use multiple dsps, as performance is easy at low spend volumes and gets more difficult as you spend more. Dsps have access to unique inventory (through direct deals, pmps, etc) and unique technologies. For example, many companies use google, a vendor for mobile, one for ctv, one for retargeting, etc.