Why shouldn't you overuse words like 'just' and 'then'?

Just and then are filler words that add little to no value to your writing. Eliminate both to tighten and strengthen your writing.

I woke up, then went downstairs to get a cup of coffee, and then I listened to the news report.

Instead, try:

I woke up, went downstairs for a cup of coffee, and listened to the news report.

Just is often redundant and unnecessary.

After work, she just ran a few errands and then headed home.

Instead, try:

After work, she ran a few errands and then headed home.