Generally speaking, the answer is no. You as the tenant enter into a lease once every so often. A broker negotiates for space on a daily basis. Would you go into an IRS audit without having your CPA by your side? The same reasoning applies here.
Keep in mind that experienced brokers know the landlords in a market well, much more than you do. They have the history and knowledge to ask for things you might not have even thought of. They might remember how much in Tenant Improvement (TI) allowance a previous tenant received and push for you to receive a similar amount. They also have an understanding of who the flexible landlords are if you think your needs might change or the landlords who will buy out your existing lease would be.
There are some cases where you might lease space on your own. The biggest driver is the dollar value of the lease commitment. The less space you lease and the shorter the term, the less rent that is due under the lease. Less rent means a smaller commission. A listing agent is much more motivated to lease space directly to a tenant versus having to share their commission. This is especially true for small spaces that generate small commissions. Also, if you are leasing a small space, you sometimes can’t get a tenant rep to work for you because it is simply not worth their time.