2. If she wins the tournament (as she should, considering the opposition and the surface) she will end up with 3555 points in the Shenzhen ranking and her place at the WTA Finals is almost guaranteed (if she does not lose everything after that, she should be able to scramble together the 300 additional points she probably needs to qualify over the coming three months).
BINGO, that's the reason.
But also this is her 17th tournament this year, so the idea is for this tournament to replace the P5 in Dubai where she only has 1 point on her 2019 Race ranking. That's why she only makes 279 points here instead of 280.
But also because this is her 17th tournament, it means everything she makes from now on will replace something else that isn't a Grand Slam, a Premier Mandatory or one of the two P5 she has to have on her ranking (at the end of the year, very important to note that detail, she will only be forced to have 2 P5 at the end of the year as a top 10 player). Also this is the last International she is allowed to have on her schedule for 2019.
So when drawing the line the results she can't replace from her current ranking are 70 points from AO, 120 and another 120 from IW and Miami, 1000 from Madrid, 70 from RG and 130 from WIM. She also has 350 from the P5 in Rome, at will need to have at least one more P5 added to her total. So yes, when USO and Beijing come she's going to drop her current two worst performing tournaments, and then at least another one replaced by a P5 (again as a top 10 player she needs at least 2 P5 on her ranking and also 2 Premiers, but she is covered as far as Premiers go).
Palermo is a good cushion for her Race total, a safety net in case she doesn't do great in either one of Toronto or Cincinnati, getting some of that pressure off of her shoulders. Even so one thing is certain, she'll need to do better at the next Slam if she wants to guarantee her spot in Shenzen (again she will drop some of her current Premiers in a few weeks, and if she doesn't do better than those points in the tournaments that replace them, she will actually lose Race points, which is something we rarely see these days).