You may have heard cautions about using chemicals on blocked drains because they're very corrosive and can cause damage to the pipes. This is certainly true, but another reason to avoid using them when a drain is blocked or flowing slowly is that they may not actually address the cause of these issues. A drain doesn't always get plugged up just due to soap scum clinging to the pipes, so note a few of those other common causes and why they should be left to a professional plumber.
1. Tree roots
Tree roots may wrap around plumbing pipes outside your home and cause cracks; these roots may then grow through that crack and block the flow of water through the pipe. Dirt and other sediment may also settle into the pipe through the crack and cause the blockage. Chemicals won't work to somehow dissolve tree roots and dirt, and even if they did, the roots would simply grow back and more dirt would settle into the pipe. It's best to have a plumber cut away the roots and redirect them or reroute new plumbing pipes outside your home as needed.
2. Foreign objects
It's not unusual for children to flush small toys and other such objects down the toilet, just to play. A person may also drop a piece of jewelry or something else into the toilet or down a sink, perhaps not even realizing it. People may also flush baby diapers, baby wipes, paper towels, and other such objects down the toilet. All these things can cause blockages in the plumbing pipes outside your home, so that drains get clogged and won't empty. Chemicals won't break these things down or somehow get the water flowing around them, so it's best to have foreign objects snaked out of pipes instead.
3. Water leaks
You may wonder how a water leak can cause a clogged drain or one that drains slowly, and this is because a water leak means that the water isn't flowing through the pipes with the pressure needed to keep drains working properly. A leak causes water to slow down its flow rate, as some of the water drips through the leaky area rather than keeping that flow of water moving smoothly. Chemicals won't address this or get the water pressure back to where it should be to get drains emptying properly, and they won't protect your home from damage from that water leak either, which is why it's best left to a plumber to address drains that don't empty as they should.
For more information and help with your blocked drains, contact an experienced plumber in your area.