I used to have the same problem as you, where I practically never used nairs or dtilts. After I started making a point to use them, nair and dtilt are some of my favorite moves.
Nair is a very safe approach because of the two hits. The first hit scares them off, and the second hit has great knockback when tippered. Nair is also a very reliable tool when gimping. Honestly, anyone could see a fair or bair coming and just airdodge that. Since a lot of players are conditioned to airdodge once Marth is within fair range, nair is a great punisher. They airdodge the first hit, but the second hit comes in and hits them - most likely killing when tipped.
Dtilt is a great move to use to set your opponent up for something else. A nontippered dtilt can even lead into a tipped dtilt. From a dtilt, you can walk into a tipped ftilt, or dash into Dancing Blade.
If you use dtilt and nair in your games, you'll definitely see some major improvements.