1) Creating a XML string by hand can become cumbersome, if XML is
huge. Maintaining the correct parent child relationship for a huge
document can be difficult, if done by hand (imagine a document of size
50 MB). This would lead to difficult debugging. Using a DOM API can do
this inherently in memory.

2) It's difficult to remember correct XML name conventions if done by hand.

for e.g., <9abc> is an invalid XML name (because it starts with a number).

There are more rules for XML names.

Using DOM API does this automatically.

3) Using DOM API can check well-formedness of entities (like, &abc;
etc). Doing this by hand in a string can become difficult.