Your plumber knows best when it comes to new installations
The other time that hiring a professional plumber is critical is when you are installing a new bathroom or adding a toilet, tub, shower or lavatory. Your plumber can advise you on which kinds of plumbing components will fit your needs best and he will know how to install them correctly.
Do you need to keep costs to a minimum? Do you want to minimize future problems? Do you need to connect new plumbing to old pipes? Do you want to be able to do repairs yourself in the future? Depending on your needs, different kinds of fixtures, pipes, joints and connectors will be appropriate. Copper pipes may be the best choice for you, or it may be that PVC or PEX tubing will work best. Should you use sweated joints or compression fittings? New plumbing technologies are being developed all the time; your plumber will know which ones are best for you.
Even more importantly, when installing new plumbing, your plumber will ensure that your plumbing is up to code and sufficient for your water supply and drainage needs. Your plumber can help you determine how much water you are likely to use on a daily basis and the kinds of supply lines and drain pipes you'll need to accommodate that.
Additionally, your plumber will make sure that drain pipes are vented properly and run correctly to prevent back-ups and other problems. Remember that most bathroom plumbing is run between walls and floors; if a leak occurs in those hidden places, it can cause massive damage to your home's framing, walls and ceilings before you are even aware that there's a problem.
For most bathroom plumbing repairs and installations, hiring a professional plumber just makes the most sense.