Tax Preparation is a useful extra to have included in a package.
- I can see it would help where you have dual reporting (US).
- It also helps where you have 'split year treatment'. I.e. In your year of arrival you're liable for tax 'back home', but also in Singapore.
- Even more so if do a transfer between non-home countries, example: you're British, hold taxable assets (you're a landlord/or you rent out your home whilst expatted), transfer to Japan, maybe for a couple of years, then transfer on to Singapore. In your year of arrival in SG that would be three separate tax returns!
I was in the latter shoes, and there's no way I could have adequately interpreted the JP and SG tax codes and filed accurately.
I had another iteration where I was still a UK landlord, and had to file back home/UK, but I was working in the US so had to do state and federal returns too*. Another three returns.
There's simply no way I could have managed such myself. And having to DIY it, or try and find an international accountant to personally instruct would have been onerous and a distraction from the job.
The solution was pretty simple. The company retained the services of their external auditors (D+T, or E+Y) to do all and any tax reporting required by their expat staff. So I visited say the SG branch of D+T, had a meeting with their specialist expat personal accountants, gave them all the base data and they coordinated all the prep and filing via their respective international offices. As simple as that.
A huge weight off my mind, and as I recall the cost to my employer was pretty minimal, as the work was regarded as incremental to their main work as auditors, and perhaps regarded as a service that was offered as something of a goodwill task.
* There are things in international tax codes you might find surprising. In the US for example, IIRC, you depreciate the value of the fabric of a
property over 40 years. Quaint! Especially as my property was all about 120+ years old

But they have ways of splitting the value of the building's fabric from the land it sits on. It gets complicated!
-- Another potential trap. In SG you pay tax at the end of the year. It would help greatly to have an estimate of what your future liability might be well before it comes liable.