What it assumes, unless you change it

This is your only income, and a full year's allowances and tax bands apply, even when the contract is shorter. For more than one contract in a year, use Add up your contracts.
You have tax code 1257L and National Insurance category A.
A limited company pays you a £12,570 salary and pays out all the rest as dividends. It does not claim the Employment Allowance and has no associated companies.
An umbrella company charges a fee of £25 a week. Apprenticeship Levy is not passed on.
An agency pays employer NI on top of an inside IR35 rate. Many agencies quote a rate that already has it taken out.
A sole trader's Class 2 NI is treated as paid.

What it leaves out

Other income, benefits in kind, savings and capital gains.
Capital allowances on equipment and vans, the trading allowance, and statutory pay such as paternity pay.
Emergency tax codes, and rules that change part-way through a tax year.
Whether your contract is inside or outside IR35. Modulas works out the tax for each. It does not decide your status.
Making Tax Digital. Modulas tells you when your income passes a threshold. This does not change your take-home.

Two rules do this, and Modulas shows them as they are. It does not smooth them.

Marriage Allowance is worth £252 and is lost in full when the recipient's income passes the basic rate band. £1 more pay can cost £251 of take-home.
A student loan counts all your dividends once they pass £2,000 in a year. A little more profit can add a repayment on the first £2,000 too.

For your own situation, talk to a qualified accountant who knows IR35 and limited companies.